Conjunctive Water Management Conceptual Study

Conjunctive Water Management Conceptual Study August 15, 2012

1697 Cole Boulevard, Suite 200

Golden, Colorado 80401

Conjunctive Water Management Conceptual Study Released

August 15, 2012

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Table of Contents List of Figures ...................................................................................................................................................... ii

List of Tables ...................................................................................................................................................... iii

List of Abbreviations .......................................................................................................................................... iv

Executive Summary ........................................................................................................................................... 1 1. Introduction and Project Understanding ................................................................................................. 1-1

1.1 Key Issues in the Central Platte River Valley ................................................................................ 1-1 1.1.1 The Platte River Recovery Implementation Program ..................................................... 1-1 1.1.2 Over-appropriation above Elm Creek .............................................................................. 1-2 1.1.3 Development of Integrated Water Management Plans ................................................. 1-2

1.2 Project Setting and Background ................................................................................................... 1-3 1.2.1 General Description of Conjunctive Water Management .............................................. 1-3 1.2.2 Conjunctive Water Management in Other States .......................................................... 1-3 1.2.3 Conjunctive Water Management Applied to CNPPID ..................................................... 1-4

1.2.3.1 Overview of the Existing System ............................................................................. 1-4 1.2.3.2 Current Conjunctive Management in the CNPPID System .................................... 1-4 1.2.3.3 Proposed Enhancements to Conjunctive Water Management at CNPPID ........... 1-5

2. Study Objectives and Tasks ..................................................................................................................... 2-1 3. Assessment of Existing Conditions .......................................................................................................... 3-1

3.1 Assessment of Access to the Groundwater Mound ..................................................................... 3-1 3.1.1 Approach and Assumptions............................................................................................. 3-1 3.1.2 Results .............................................................................................................................. 3-3

3.2 Hydrogeologic Conditions of the Aquifer ...................................................................................... 3-4 3.3 Assessment of Infrastructure ........................................................................................................ 3-4

3.3.1 Historical Losses from CNPPID Canals and Laterals ..................................................... 3-4 3.3.2 Spatial Distribution of Groundwater Recharge from Canals ......................................... 3-5 3.3.3 Recharge Strategies ........................................................................................................ 3-5

3.4 Water Budget Development .......................................................................................................... 3-7 3.4.1 Process for Estimating Net Irrigation Requirement ....................................................... 3-7 3.4.2 Water Budget Results ...................................................................................................... 3-7

4. Assessment of Effects and Benefits of Conjunctive Water Management ............................................ 4-1 4.1 Modeling Tools ............................................................................................................................... 4-1

4.1.1 OPSTUDY .......................................................................................................................... 4-1 4.1.1.1 Modifications to OPSTUDY ...................................................................................... 4-2 4.1.1.2 Description of OPSTUDY Modeling Process ........................................................... 4-2

4.1.2 COHYST ............................................................................................................................. 4-3 4.1.2.1 Integration of groundwater modeling. .................................................................... 4-3

4.2 Assumptions ................................................................................................................................... 4-4

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4.3 Conjunctive Water Management Scenarios ................................................................................ 4-5 4.3.1 Operational Goals of Scenarios ...................................................................................... 4-5 4.3.2 Scenario Descriptions ..................................................................................................... 4-6 4.3.3 Scenario 1 Simulation Results ....................................................................................... 4-7

4.3.3.1 Surface Water and Hydropower Effects ................................................................. 4-7 4.3.3.2 Groundwater Effects ............................................................................................... 4-9

4.3.4 Scenario 2 Simulation Results ..................................................................................... 4-10 4.3.4.1 Surface Water and Hydropower Effects ............................................................... 4-10 4.3.4.2 Groundwater Effects ............................................................................................. 4-11

4.3.5 Scenario 3 Simulation Results ..................................................................................... 4-12 4.3.5.1 Surface Water Effects ........................................................................................... 4-12

5. Conclusions and Recommendations ...................................................................................................... 5-1 5.1 Conclusions .................................................................................................................................... 5-1 5.2 Recommendations ........................................................................................................................ 5-2

6. Limitations ................................................................................................................................................ 6-1

References .................................................................................................................................................. REF-1

List of Figures Figure 1-1. Study Area Overview Map

Figure 3-1. Irrigation Well Needs in Main CNPPID Service Area

Figure 3-2. Annual Delivery Losses and Delivery Efficiency for CNPPID Irrigation System

Figure 3-3. Summary of Average 1954-2002 Historical Monthly Delivery Loss Amounts from CNPPID Irrigation Distribution System

Figure 3-4. Assessment of Capabilities of Open Canals and Laterals to Distribute Recharge and Recharge Strategies

Figure 3-5. Summary of CNPPID Water Budget

Figure 4-1. Use of Groundwater Mound in Combination with Lake McConaughy Storage under Conjunctive Water Management Scenario 1

Figure 4-2. Effects of Conjunctive Water Management Scenario 1 on Annual Flows at Grand Island

Figure 4-3. Effects of Conjunctive Water Management Scenario 1 on Flows at Grand Island During Times of Shortage

Figure 4-4. Effects of Conjunctive Water Management Scenario 1 on Average Monthly Flows at Grand Island During Times of Shortage

Figure 4-5. Mounding/Decline Potential at the End of Scenario 1 Simulation

Figure 4-6. Use of Groundwater Mound in Combination with Lake McConaughy Storage under Conjunctive Water Management Scenario 2

Figure 4-7. Effects of Conjunctive Water Management Scenario 2 on Annual Flows at Grand Island

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Figure 4-9. Effects of Conjunctive Water Management Scenario 2 on Average Monthly Flows at Grand Island During Times of Shortage

Figure 4-10. Mounding/Decline Potential at the End of Scenario 2 Simulation

Figure 4-11. Additional Increase in Flow at Grand Island (Beyond Scenario 1 Improvements) During Times of Shortage Due to Additional Storage of Various Capacities


List of Tables Table 3-1. Water Budget for CNPPID System ............................................................................................... 3-8

Table 4-2. Operational Goals for Conjunctive Water Management Scenarios .......................................... 4-6

Table 4-3. Description of Conjunctive Water Management Scenarios ....................................................... 4-7

Table 4-4. Summary of Scenario 1 Stream Flow and Hydropower Effects ................................................ 4-8

Table 4-5. Summary of Scenario 2 Stream Flow and Hydropower Effects .............................................. 4-10

Table 5-1. Summary of Potential Conjunctive Water Management Program Benefits ............................. 5-1

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List of Abbreviations

AF Acre-feet

CALMIT Center for Advanced Land Management Information Technologies

CNPPID Central Nebraska Public Power and Irrigation District

COHYST Cooperative Hydrology Study

CPNRD Central Platte Natural Resources District

CPR Model Central Platte River OPSTUDY model

EIS Environmental Impact Statement

FWS U.S. Fish and Wildlife Service

IMP Integrated Surface Water and Groundwater Management Plan

NDNR Nebraska Department of Natural Resources

NIR Net irrigation requirement

NPPD Nebraska Public Power District

NRD Natural Resources District

PRRIP Platte River Recovery Implementation Program

TPNRD Twin Platte Natural Resources District

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Executive Summary Brown and Caldwell evaluated the conceptual-level feasibility of operating the surface water irrigation system owned and managed by the Central Nebraska Public Power and Irrigation District (CNPPID) to enhance the conjunctive use and overall utilization of surface and groundwater supplies. This summary highlights the Conjunctive Water Management Conceptual Study (Conceptual Study).

Nebraska faces many challenges in stretching its limited water supplies to meet a wide variety of demands. The Platte River in central Nebraska is a focal point for many of the complex water supply and demand issues facing the state. Large water demands for irrigation, power production, environmental, and other uses are sometimes not completely satisfied because of water supply variability and insufficient water storage and supply facilities to meet demands during drier hydrologic periods. In fact, environmental demands for Platte River flows are often not completely satisfied during normal hydrologic periods. The shortage of water supplies has induced many regulatory and legislative actions in Nebraska over the last two decades including establishment of the Platte River Recovery Implementation Program, the designation of the Platte River (including the North Platte and South Platte Rivers in Nebraska) above Elm Creek as being over-appropriated, and the requirement for the development of Integrated Surface and Groundwater Management Plans. Conjunctive water management is a tool that can be used to help meet the requirements of these regulatory and legislative actions.

Conjunctive Water Management

Conjunctive water management generally involves the coordinated use of surface water supplies and storage with groundwater supplies and subsurface storage. In practical terms, conjunctive water management involves using surface water supplies when they are plentiful such as wet weather years, and using groundwater supplies during drier times such as drought years and summer/irrigation seasons. When available, excess surface water supplies are captured and infiltrated into the aquifer to replenish groundwater supplies. The groundwater supplies are then stored and used to meet demands during drier periods. Conjunctive management provides an increase in the overall storage of water supplies, allowing for more use of the total water supply. Conjunctive water management is a tool already in use by surface water irrigators in Nebraska who drill groundwater wells to supplement surface water supplies. In wet years when stream flows are high, these irrigators rely primarily on their surface water supplies for irrigation. In drier years, the same irrigators will utilize the wells to provide groundwater supplies.

The actions described in this report would increase the level of intentional conjunctive water management at CNPPID. During drier times when stream flows are not as plentiful, the groundwater mound under CNPPID’s service area would be the primary source of irrigation supply for CNPPID irrigators. During wet hydrologic cycles, the mound would be recharged using available and excess stream flow. The system of canals and laterals would be used primarily as recharge facilities rather than facilities for irrigation water delivery. CNPPID irrigators would require groundwater wells so that they could access the groundwater mound and the recharge delivered through the canal system. Enhanced conjunctive water management offers benefits such as greater reliability of supply to CNPPID irrigators, additional operational flexibility, increased flows in the Platte River during times of shortage, greater stability of water levels in Lake McConaughy, and reduced non-beneficial water consumption.

Executive Summary

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Study Goal and Objectives

The overall goal of this study was to technically evaluate at a conceptual-level, the feasibility and potential benefits of enhancing conjunctive water management activities in CNPPID’s service area. The objectives evaluated to meet the goal of the study included:

Investigate feasibility of utilizing the canal and lateral system to enhance groundwater recharge rather than direct deliveries of surface water for irrigation.

Understand the potential effects of enhanced conjunctive water management on hydropower production.

Estimate potential Platte River flow increases resulting from conjunctive water management.

Identify potential recreational and flood control benefits at Lake McConaughy resulting from conjunctive water management.

Estimate non-beneficial consumptive use savings that could be achieved.

To meet the study objectives, the following tasks were conducted.

Identify lands without irrigation wells. Identify areas with inadequate aquifer.

Assess the existing delivery system’s ability to recharge.

Develop a water budget. Assess the effects and benefits of conjunctive water management.

Assessment of Existing Conditions

The study tasks included in the assessment of existing facilities/conditions were identification of lands without irrigation wells, identification of areas with inadequate aquifer (if any), assessment of the existing delivery system’s ability to provide recharge, and development of a water budget. The approach and results for each task are described below.

Assessment of Access to the Groundwater Mound

The approach for this task relied on maps of several features including irrigated lands, surface water irrigated lands and registered, active irrigation wells. GIS was used to overlay the mapping layers and to identify potential fields that would need irrigation wells. The mapped data used for the assessment included irrigated lands, surface water irrigated lands, and registered active irrigation wells.

Using the process and assumptions described in Section 3.1.1 of the report, the number of wells needed to provide groundwater to CNPPID irrigators that are wholly dependent on surface water was estimated to be approximately 450 wells. The assessment also showed that approximately 75% of CNPPID lands already have irrigation wells, which is consistent with recent reports from CNPPID.

Hydrogeologic Conditions of the Aquifer

The ability of the regional aquifer to yield suitable quantities of water as well as absorb significant recharge in the vicinity of the CNPPID service area was evaluated in a qualitative manner using the existing distribution of groundwater wells and hydraulic parameter distribution from the COHYST Eastern Model Unit groundwater model.

The present day distribution of irrigated land with access to groundwater wells suggests that significant groundwater resources are currently being utilized within the CNPPID service area. Additionally, the surrounding agricultural lands immediately adjacent to the CNPPID service area are served by an extensive distribution of groundwater wells. The large number and spatial distribution of groundwater production wells supports the conclusion that the regional aquifer is viable and capable of supporting pumping for irrigation purposes. The only exception to this conclusion is the area in the immediate

Executive Summary

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vicinity of the E-67 canal system, which has historically experienced poor hydrogeologic conditions (CNPPID, 2011).

The distribution of hydraulic parameters from the COHYST model suggests that significant groundwater resources are available throughout the CNPPID service area, as well as immediately adjacent to their serviced lands. Additionally, given the degree of groundwater rise within the vicinity of the CNPPID service area over the past 50 years, it is likely that groundwater reserves within and adjacent to the CNPPID service area have been supported not only by suitable hydraulic aquifer conditions, but also a consistent supply of groundwater recharge. This confluence of favorable conditions strongly supports that the hydrogeologic conditions of the Ogallala aquifer within the CNPPID service area are adequate for continued and additional development as a source of irrigation water supply as well as a receptor for future planned recharge and replenishment efforts. In areas closer to the Platte River, depths to the water table are more shallow and have a smaller volume of storage potential in the vadose zone which will likely limit recharge potential in these areas.

Assessment of Infrastructure

A high-level analysis of historical delivery losses from CNPPID canals and laterals was conducted to derive a general understanding of annual and monthly magnitudes of losses. Annual estimates of seepage loss and delivery efficiency were developed based on data from CNPPID for the 1954 to 2007 time period. Overall system losses averaged approximately 140,000 AF per year from 1954 to 2007, but in recent time periods (after the early 1980s), annual delivery losses ranged from 100,000 to 120,000 AF per year. Monthly estimates of delivery losses were also developed, and maximum losses tend to occur in July and August with an average of approximately 40,000 AF per month. The delivery loss estimates show that the existing canal and lateral system have a large capacity to provide recharge to the aquifer.

Spatial Distribution of Groundwater Recharge from Canals

To estimate the impact of recharge from existing canals and infrastructure, a mapping-based assessment was conducted assuming a radial influence of 1 mile for potential hydraulic response, or “mounding”, around open canals and laterals within the CNPPID service area. It was assumed that, over the short term, aquifer levels within 1 mile of a canal or lateral would increase due to seepage when water is being delivered through the canal or lateral. In general, the mapping-based assessment described above supports the conclusion that the network of existing CNPPID canals and laterals are well distributed throughout the CNPPID service area and could provide recharge to most suitable locations. In areas where open canals and laterals do not currently exist, it is possible that additional, engineered recharge facilities may need to be constructed.

Recharge Strategies

Given the estimates of seepage loss from the existing canals and laterals from the 1954 to the 2007 time period and the assumed radius of influence of hydraulic response, or mounding, from future potential groundwater recharge, three classes of groundwater recharge were identified as potential strategies for future, conjunctive management scenarios. These strategies are listed and described below:

Distributed Recharge, which refers to the use of existing canals, laterals, and field application to spread a given volume of groundwater recharge over a large areal extent.

Localized Recharge, which refers to the use of engineered, rapid infiltration basins and underground injection wells to emplace groundwater recharge in high volumes in localized areas.

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Recharge and Recovery, which refers to managed recharge and groundwater extraction using existing canal and lateral infrastructure and current as well as future pumping wells to maintain an optimal depth to groundwater for crops as well as irrigation wells.

Three representative areas for application of the above recharge strategies were identified given the assessment criteria of 1) current depths to groundwater and proximity to perennial drainages associated with the Platte River, 2) proximity of existing recharge infrastructure, and 3) future, estimated mounding potential associated with existing canals and laterals. However, these recharge strategies may be applied where appropriate throughout the CNPPID service area and adjacent region depending upon land availability, localized hydrogeologic conditions, and groundwater demands. Representative areas where recharge strategies may be applied are summarized below:

Along the E-65 canal where depths to groundwater and associated, underground water storage potential are greater than those immediately adjacent to the Platte River, distributed and localized recharge are both viable options for conjunctive water management.

Immediately south of the central portion of the CNPPID service area, between the E-65 and eastern extent of the Phelps canal is an area that may be suitable for higher volume, localized groundwater recharge operations.

The eastern portion of the CNPPID service area served by the Phelps canal is suitable for localized groundwater recharge and recovery operations. Aquifer conditions in this area are characterized by shallower depths to the water table and are more susceptible to negative impacts to agricultural fields from high water tables as well as seepage into subsurface domestic structures. A managed recharge and extraction regime in this area would not only help optimize conjunctive water management, but could also improve waterlogged conditions (to the extent that they exist) in the eastern extent of the CNPPID service area via managed groundwater extraction and management of seasonal water table elevations.

Water Budget Development

A water budget was developed for CNPPID’s system as a guideline for creating and evaluating conjunctive water management scenarios. The water budget also served as a useful tool for understanding the relative magnitude of water volumes diverted, returned, seeped, and consumed. Data for the water budget was obtained from NDNR gaging records, CNPPID records of diversions into the irrigation system and on-farm deliveries, and estimates of net irrigation requirements (NIR) derived using output from the CROPSIM model.

The table below summarizes the water budget and shows the data source and timeframe over which averages were calculated for each water budget component. Water budget data were estimated on a system-wide basis and for the irrigation system.

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Water Budget for CNPPID System

Component Amount (AF/year)

Data Source Timeframe

Syst

em-W

ide

Total Annual Diversions into the Tri-County Supply Canal

1,074,000 NDNR gage 142000 1954-2008

Returns through Jeffrey 47,000 NDNR gage 143000 1954-2008

Returns through J-2 547,000 NDNR gage 144000 1954-2008

Seepage and Lake Evaporation Prior to Delivery to Irrigation System

257,000 Calculated remainder

Amount Provided to Irrigation System 235,000 CNPPID records 1954-2007

Irrig

atio

n Sy

stem


Seepage Loss in Canals and Laterals 138,000 CNPPID records 1954-2007

Historical CNPPID On-farm Irrigation Deliveries

97,000 CNPPID records 1954-2007

Net Irrigation Requirement 71,000 Processed CROPSIM output

1954-2005

A summary of the “fate” of water that is diverted into the CNPPID system at the Tri-County Supply Canal Dam is listed below.

Approximately 55% of diversions are returned to the Platte River through the Jeffrey and J2 returns. Approximately 24% of diversions seep into the aquifer or evaporate prior to reaching the irrigation

canals and laterals.

Approximately 15% of diversions seep into the aquifer or evaporate while being conveyed in irrigation canals/laterals or is not consumed on-farm because of irrigation inefficiencies.

Approximately 6% of diversions are consumed by crops.

Assessment of the Effects and Benefits of Conjunctive Water Management

A conceptual-level assessment of the potential effects and benefits of various conjunctive water management scenarios was conducted using available surface and groundwater modeling tools. Individual surface water and groundwater modeling tools were used to assess the effects and benefits of the conjunctive water management program described in this report. The surface water modeling was conducted using OPSTUDY (a surface water modeling software package developed by the U.S. Bureau of Reclamation), and the groundwater modeling was done using the Eastern Model Unit of the COHYST groundwater model (a MODFLOW flow model). A description of the models, modifications to the models, and the modeling process are provided in subsequent sections of this report.

Modeling Assumptions

Several assumptions were used in the creation of conjunctive water management scenarios and the development of modeling inputs. The assumptions are listed below. See subsequent sections of this report for further description of these assumptions. The E-67 system would not be included in a conjunctive water management program.

Improved irrigation efficiency will result in less non-beneficial consumption.

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Recharge diversions will cover canal/lateral seepage and on-farm net irrigation requirements.

Historical contributions to the Republican River basin will be maintained. Maintenance flows may be necessary

The value of additional hydropower would be between $0.026 and $0.040 per kWh.

Description of Conjunctive Water Management Scenarios

Each of the conjunctive water management scenarios developed for this study had several overall operational goals. The operational goals are described in the table below:

Operational Goals for Conjunctive Water Management Scenarios

Goal Description

No negative impacts to CNPPID water users A primary goal of the proposed conjunctive water management program is to provide benefits CNPPID water users in terms of increased reliability of irrigation water supplies through management of the groundwater mound and Lake McConaughy.

Reduce shortages of stream flow at Grand Island Through retiming of diversions and using both surface water and groundwater storage, the conjunctive water management program should provide additional stream flow in the central Platte River during times of stream flow shortage.

Beneficially use stored groundwater in the mound By relying exclusively on groundwater during drier hydrologic cycles, the groundwater mound will be used directly by CNPPID irrigators and indirectly as a means to provide additional stream flows. In areas with high groundwater tables, lowering the level of the groundwater mound could benefit agricultural producers.

Alter operation of Lake McConaughy for additional benefits

Each of the modeling scenarios sought to maintain storage amounts in Lake McConaughy between 800,000 AF and 1,200,000 AF. Currently, Lake McConaughy storage levels are generally managed higher than this. The goal of storing less water in Lake McConaughy could result in the following:

• Enhanced recreational opportunities by maintaining shoreline access and stable water levels.

• Additional flood control by preserving reservoir storage space to capture high flows.

• Reduced evaporation losses due to decreased surface area of the lake.

Maintain or increase hydropower production Revenues from hydropower production are critical and should be either maintained or increased by the conjunctive water management program.

Additional benefits to NPPD NPPD has the first right to store inflows into Lake McConaughy at the beginning of the storage season. Once NPPD’s storage amount of 125,000 AF has been filled, CNPPID and the Environmental Account (a storage account held by the U.S. Fish and Wildlife Service) can fill. The conjunctive water management program proposed in this study does not seek to alter NPPD’s storage right in any way. Through more reliance on the groundwater mound and the potential to avoid large declines in storage levels in Lake McConaughy, the conjunctive water management program could improve the supply of cool water for Gerald Gentleman Station and enhance the reliability of supply to the Kearney Canal.

Three scenarios of conjunctive water management were developed and were evaluated using the modeling tools employed for this study. Scenarios 1 and 2 were developed as “bookends” to examine effects and benefits of conjunctive water management using nearly the entire irrigation distribution system (Scenario 1) and only a part of it (Scenario 2). Scenario 3 incorporated potential effects and benefits of additional storage upstream of Grand Island. Descriptions of the scenarios are provided in the table below:

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Description of Conjunctive Water Management Scenarios

Scenario Description Sc

enar

io 1

E-65 and Phelps systems in service

The first scenario assumed that recharge deliveries would be distributed using the E-65 and Phelps Canals and associated laterals. The scenario assumed that no deliveries for direct irrigation would be provided in these systems.

Scen

ario

2

Only the E-65 system is used The second scenario assumed that the E-65 Canal and associated laterals would be used to distribute recharge in part of the CNPPID service area. It was assumed that the Phelps Canal would not be needed for distribution of recharge or direct irrigation deliveries, because the water table is relatively high in the eastern portion of the CNPPID service area. CNPPID irrigators under the Phelps Canal would rely on groundwater wells for irrigation supply. Under the E-65 system, groundwater levels are generally lower, and there are more opportunities to recharge. Stream flow gains were reduced to account for the lack of delivery into the Phelps Canal system.

Scen

ario

3

An additional storage facility is constructed upstream of Grand Island

The third scenario used the results of Scenario 1 to assess additional stream flow benefits that could result from the addition of a new surface water storage facility upstream of Grand Island. The purpose of the storage facility would be to capture additional excess stream flow and to fine tune management of shortages and excess. The storage facility could be an off-channel reservoir, an aquifer storage and recovery facility, an existing structure such as Elwood Reservoir (with new infrastructure), etc. The specific location of the storage facility was not specified, and the analysis assumed that the facility would be a surface water reservoir. The evaluation of Scenario 3 was conducted using a spreadsheet tool rather than OPSTUDY and the COHYST model.

Simulation Results

The results of Scenario 1 and 2 simulations with respect to stream flows and hydropower are described in the table below. Further description and explanation of the parameters and output is provided in subsequent sections of the report. In summary, both Scenario 1 and 2 provided enhancements to stream flow at Grand Island, especially during times of shortage. In addition, simulations of both scenarios showed maintained or increased levels of hydropower production.

Summary of Scenario 1 and 2 Stream Flow and Hydropower Effects

Parameter Change under Conjunctive Water Management Program – Scenario 1

Change under Conjunctive Water Management Program – Scenario 2

Stream Flow

Average Annual Platte River Flow at Grand Island Increase of 31,000 AF/yr

(46,000 AF/yr with Environmental Account)

Increase of 64,000 AF/yr


Average Annual Platte River Flow at Grand Island during Times of Shortage





Hydropower Output

Kingsley -9.7 MKWH/yr (decrease) -6.5 MKWH/yr (decrease)

North Platte Hydro +1.7 MKWH/yr (increase) +4.2 MKWH/yr (increase)

Central Supply Canal (Jeffrey, J1, J2) +8.1 MKWH/yr (increase) +15.7 MKWH/yr (increase)

Total +0.1 MKWH/yr (slight increase) +13.4 MKWH/yr (slight increase)

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The potential for long term mounding and groundwater table decline was evaluated using the COHYST model on a relative, and not an absolute basis. Factors such as variations in groundwater pumping from wells outside of CNPPID or groundwater inflows or outflows from creeks and drains that would intercept groundwater were ignored for this simplified analysis.

In Scenario 1, after 50 years of pumping to meet the full crop consumptive use demands as well as recharging the groundwater mound, results from the COHYST model show no regions of groundwater declines within the CNPPID service area over the 50-year simulation time period.

In Scenario 2, at the end of the 50-year modeling period, the COHYST model did not simulate groundwater declines in the immediate vicinity of the E-65 canal system, but it did suggest potential long term declines within the groundwater mound in the vicinity of the eastern Phelps Canal system. The integrated conjunctive use modeling suggests that some level of recharge will need to occur in the Phelps Canal service area to maintain the groundwater mound; however, it is possible to manage groundwater elevations at beneficial levels that will assist with waterlogged conditions.

Scenario 3 used the results of Scenario 1 to evaluate the potential benefits of an additional storage facility upstream of Grand Island. The analysis was conducted at a very conceptual level using a spreadsheet tool to simulate the operations of the potential storage facility. The spreadsheet tool assumed that water would be stored in the facility during months when the results of Scenario 1 showed stream flow excesses at Grand Island. Water was then released from the facility when flows at Grand Island were below FWS targets. Facilities with capacities of 40,000 AF and 250,000 AF were assessed using the spreadsheet tool. On an average annual basis, the 40,000 AF storage facility provided an additional 14,000 AF of stream flow improvement during times of shortage at Grand Island. The 250,000 AF facility provided an average of approximately 50,000 AF/yr of additional flow during times of shortage.

Conclusions

This study assessed the potential to use existing infrastructure for recharge, evaluated infrastructure needed to conduct a conjunctive water management program (i.e. new groundwater wells, additional recharge facilities, etc.) and estimated the effects and benefits of various conjunctive water management scenarios. The conclusions of this conceptual-level study indicate that the conjunctive water management program described in this report could greatly benefit CNPPID, stakeholders in the central Platte River valley, and the State of Nebraska. The table below summarizes the potential benefits of the conjunctive water management scenarios.

Summary of Potential Conjunctive Water Management Program Benefits

Parameter Benefits of Conjunctive Water Management

Average annual Platte River flow at Grand Island

Increase of 30,000 to 80,000 acre-feet/year

Average annual Platte River flow at Grand Island during times of shortage


(Potentially over 215,000 acre-feet/year with a 250,000 acre-foot storage facility)

Total Hydropower Generation Increase of 0.1 to 13.4 MKWH

(Increased revenues of potentially over $500,000 per year on average)

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Several additional conclusions can be drawn from the results of the evaluations, including: CNPPID irrigators can reliably use the groundwater mound for irrigation supply. By using the

groundwater mound as the primary source of water supply, CNPPID irrigators would be able to irrigate when needed and not be subject to potential surface water supply issues.

Existing infrastructure can be used for conjunctive water management. The existing canal and lateral system would be the primary recharge facility in a conjunctive water management program.

Increased hydropower revenues could be realized. OPSTUDY modeling indicates that overall hydropower production in the central Platte River valley could increase under a conjunctive water management program.

Lake McConaughy water levels could be managed for greater stability. By relying on the groundwater mound as another reservoir of irrigation water supply, CNPPID could gain additional flexibility in managing storage in Lake McConaughy, which could lead to greater stability in water levels.

Water can be provided to help return the Platte River below Lake McConaughy to fully appropriated conditions. The stream flow benefits derived from a conjunctive water management program would be very beneficial for returning to fully appropriated status in the Platte River basin.

The PRRIP could benefit significantly from this program. Surface water modeling conducted for this study suggested that significant stream flow increases could be realized during times of shortage.

Recommendations

The proposed conjunctive water management program warrants more research and collaboration among parties that might participate in or benefit from the program. Recommendations for future activities to advance this program are listed below: Refine the modeling analysis. This study was conducted at a conceptual level and the analysis tools

had several limitations. Refinement of the model would improve the accuracy of estimated results.

Develop operating rules. The modeling analyses conducted for this study were based on several assumptions regarding how the conjunctive water management program would be operated. CNPPID, NPPD, the PRRIP, and other stakeholders who could benefit or be impacted by the program should be engaged and should collaborate to develop and refine program objectives and operating rules.

Assess current and future infrastructure needs. Additional assessments of needed infrastructure should be conducted once more refined modeling is performed and operating rules and goals are developed

Explore legal, environmental, and socioeconomic considerations. Legal, environmental, socioeconomic and other considerations should be evaluated as the conjunctive water management program develops

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Section 1

Introduction and Project Understanding Brown and Caldwell evaluated the conceptual-level feasibility of operating the surface water irrigation system owned and managed by the Central Nebraska Public Power and Irrigation District (CNPPID) to enhance the conjunctive use and overall utilization of surface and groundwater supplies. This report describes Brown and Caldwell’s Conjunctive Water Management Conceptual Study (Conceptual Study) and includes the following topics:

Water management issues and programs in the Platte River basin and potential benefits derived from this conjunctive water management program (Section 1)

Study objectives and tasks (Section 2)

Assessment of existing and needed infrastructure and its potential use in a conjunctive water management program (Section 3)

Benefits of conjunctive water management under different operational scenarios (Section 4)

Study conclusions, limitations, and recommendations (Section 5)

1.1 Key Issues in the Central Platte River Valley Nebraska faces many challenges in stretching its limited water supplies to meet a wide variety of demands. The Platte River in central Nebraska is a focal point for many of the complex water supply and demand issues facing the state. Large water demands for irrigation, power production, environmental, and other uses are sometimes not completely satisfied because of water supply variability and insufficient water storage and supply facilities to meet demands during drier hydrologic periods. In fact, environmental demands for Platte River flows are not totally satisfied even during normal hydrologic periods.

The shortage of water supplies has caused many regulatory and legislative actions in Nebraska over the last two decades. Several water management programs, including those described in the following sections, were the result of these actions.

1.1.1 The Platte River Recovery Implementation Program

The Platte River is a highly-utilized source of water supply for agricultural, hydropower and other uses, but it is also critical habitat to threatened and endangered species including the threatened piping plover and the endangered whooping crane, interior least tern, and the pallid sturgeon. Potential litigation associated with the protection of these species and the relicensing of Kingsley Dam in the early

Section 1 Introduction and Project Understanding

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1990s drove officials in the Platte River basin states and the federal government to work towards a cooperative strategy for addressing water supply issues.

In 1997, the Secretary of the Interior and the governors of Wyoming, Colorado, and Nebraska signed the “Cooperative Agreement for Platte River Research and Other Efforts Relating to Endangered Species Habitat along the Central Platte River, Nebraska” (Cooperative Agreement). The Cooperative Agreement paved the way for the states and the federal government to develop a program to enhance habitat for threatened and endangered species in the central Platte River. Through the Cooperative Agreement, the Platte River Recovery Implementation Program (PRRIP) was developed and began in 2007.

The PRRIP will take an incremental approach to increasing stream flows in the central Platte River, enhancing habitat for target bird species, and accommodating new water-related activities. In the first increment of the PRRIP, which will take place from 2007 to 2019, target flow shortages will be reduced by 130,000 to 150,000 acre-feet (AF) per year by retiming and increasing river flows. To reach this goal, the PRRIP and participating states have implemented several projects to retime and increase flows, and they are actively studying and pursuing additional projects.

1.1.2 Over-appropriation above Elm Creek

In 2002, the Nebraska Legislature created the Water Policy Task Force to study issues related to Nebraska’s laws governing surface water and groundwater. The Water Policy Task Force recommended several actions that were reflected in new water policy legislation, commonly known as LB 962. LB 962 was signed into law in April, 2004 and contained several provisions designed to anticipate and prevent conflicts between surface and groundwater users.

Several areas in Nebraska were undergoing water-related conflicts at the time the bill was signed. Areas in conflict were designated by the Nebraska Department of Natural Resources (NDNR) as “over-appropriated,” meaning the extent of water development in these areas is not sustainable over the long term. The Platte River basin above the Kearney Canal diversion near Elm Creek, Nebraska was designated as over-appropriated. The designation impacted the Twin Platte Natural Resources District, the North Platte Natural Resources District, the South Platte Natural Resources District and portions of the Central Platte Natural Resources District and the Tri-Basin Natural Resources District.

LB 962 also included provisions related to “fully appropriated” areas, annual evaluations of water supplies and demands, stays on new uses of groundwater and surface water, and the development of Integrated Surface Water and Groundwater Management Plans (IMPs). The reader is encouraged to review other references for a full description of the provisions in LB 962.

1.1.3 Development of Integrated Water Management Plans

Natural Resources Districts (NRDs) designated as “over-appropriated” and NDNR are required by LB 962 to jointly develop an IMP within 3 to 5 years of the designation. Several requirements for IMPs were described in LB 962 including the following:

Clear goals and objectives with a purpose of sustaining a balance between water uses and water supplies so that the economic viability, social and environmental health, safety, and welfare of the river basin, sub-basin, or reach can be achieved and maintained for both the near term and the long term

One or more of the groundwater controls authorized for adoption by natural resources districts

Groundwater controls authorized by LB 962 included rotational use of groundwater, well spacing requirements, reductions in groundwater irrigated acreage, installation of measurement devices, and limits on expansion of irrigated acreage among other controls.

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NRDs in the Platte River basin upstream of and including the CPNRD have developed and are implementing IMPs. In addition, a basin-wide IMP was developed and implemented for over-appropriated portions of the Platte River basin. The NRD-specific plans include water management activities to be taken in both fully and over-appropriated areas. In over-appropriated areas, NRDs are required to reduce stream flow depletions from groundwater irrigation to levels that occurred prior to 1997 (the year in which the Cooperative Agreement was signed) during the first 10 years of the IMP. The long term goal of the IMPs is to return over-appropriated areas to fully appropriated or even under-appropriated conditions.

1.2 Project Setting and Background Creative thinking has been and will be needed to meet the requirements of the PRRIP and IMPs described above. Water managers and water users have been developing innovative projects over the last decade to meet the goals of the PRRIP and, more recently, IMPs. Many of these projects have involved conjunctive water management.

1.2.1 General Description of Conjunctive Water Management

Conjunctive water management was defined concisely in Common Waters, Diverging Streams (Blomquist, et al., 2004) as the following:

Conjunctive water management involves the coordinated use of surface water supplies and storage with groundwater supplies and storage.

In practical terms, conjunctive water management many times involves using surface water supplies when they are plentiful during wet periods and using groundwater supplies during drier times. When available, excess surface water supplies would be captured and infiltrated into the aquifer to replenish groundwater supplies used to meet demands during drier periods. Using conjunctive management, more of the overall water supply can be utilized by capturing, retiming, and making better use of excess supply. Blomquist, et al. (2004) describe the interaction of surface water and groundwater supplies in conjunctive water management as:

Conjunctive water management can be understood as an effort to use the relative advantages of surface water and groundwater resources to offset each other’s shortcomings.

Conjunctive water management is a tool commonly used by water managers and water users in Nebraska and other western U.S. states. For example, surface water irrigators in western Nebraska often drill groundwater wells to supplement surface water supplies. In wet years when stream flows are high, irrigators may rely primarily on their surface water supplies for irrigation. In drier years, irrigators may rely more on groundwater supplies. Seepage from the surface water distribution system recharges the aquifer and is the primary mechanism for replenishing groundwater supplies that are used for irrigation.

Conjunctive water management is currently being evaluated through the Cooperative Hydrology Study (COHYST) in the central Platte River valley by the NDNR, Nebraska Public Power District (NPPD), CNPPID, Nebraska Game and Parks Commission, TPNRD, and CPNRD. The study’s objectives are to optimize the management of surface water and groundwater supplies to deliver greater overall benefits to water users. The study team is currently developing modeling and other tools that will be used to evaluate conjunctive water management strategies.

1.2.2 Conjunctive Water Management in Other States

In Colorado, irrigators commonly use managed aquifer recharge programs to divert and infiltrate excess stream flows to offset stream flow depletions caused by groundwater pumping for irrigation. Water providers in the Denver metropolitan area are studying ways to share water infrastructure to help reduce

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reliance on dwindling groundwater supplies from the Denver Basin Aquifer by sharing surface water supplies during wet years and potentially recharging the Denver Basin Aquifer.

Other western U.S. states, including California, Arizona, Idaho, etc. frequently use infiltration basins to “bank” excess stream flows for later withdrawal for municipal, industrial, and irrigation purposes. In some instances, irrigators with surface and groundwater supplies have been encouraged to rely solely on groundwater during dry times so that municipalities can make use of surface water supplies. These are just a few examples of the ways that conjunctive water management is applied in other states.

Conjunctive water management strategies may differ from state to state depending on laws governing surface water and groundwater supplies, hydrologic conditions, hydraulic connections between streams and aquifers, etc. Strategies used in other states may not be directly applicable because of institutional and hydrologic conditions in Nebraska.

1.2.3 Conjunctive Water Management Applied to CNPPID

1.2.3.1 Overview of the Existing System

CNPPID provides water for a variety of demands including surface water irrigation, hydropower, groundwater recharge, and recreation. Surface water for irrigation is supplied directly from CNPPID to approximately 112,000 acres in central Nebraska. CNPPID’s irrigation and hydropower water supply is stored in Lake McConaughy. In addition, NPPD stores water in Lake McConaughy for irrigation and hydropower uses.

Water management along the North Platte and Platte Rivers is a complex task and requires daily coordination among CNPPID, NPPD, and other water managers. Figure 1-1 shows the irrigated lands served by CNPPID, Lake McConaughy, and several irrigation and water supply facilities located between Lake McConaughy and the irrigated lands directly served by CNPPID. The following bullets generally describe how CNPPID water is conveyed from Lake McConaughy to irrigated lands directly served by CNPPID. NPPD water is conveyed in facilities upstream of and including Jeffrey Reservoir, and flows passing through various facilities may be a combination of NPPD and CNPPID water. CNPPID releases from Lake McConaughy can be diverted into NPPD’s Sutherland Supply Canal or can

be passed down the North Platte and Platte Rivers.

Water diverted into NPPD’s Sutherland Supply Canal is used for cooling at the Gerald Gentleman Station (NPPD’s 1,400 MW coal-fired power plant), recreation, and hydropower (at NPPD’s North Platte Hydro).

NPPD’s Sutherland system returns water to the South Platte River just upstream of CNPPID’s Tri-County Supply Canal diversion.

The Tri-County Supply Canal diverts water at the confluence of the North and South Platte Rivers and delivers water to CNPPID’s three hydropower facilities (the Jeffrey, Johnson No. 1 and Johnson No. 2 hydroplants), several storage reservoirs, and irrigated fields. The larger reservoirs on CNPPID’s canal system include Jeffrey Reservoir, Johnson Lake, and Elwood Reservoir.

Water can be delivered back to the river after passing through CNPPID’s hydropower facilities via the Jeffrey and J2 Returns.

Most of the irrigation supplies are delivered to lands served by CNPPID via three canals (the Phelps, E-65, and E-67 Canals) located at the eastern end of the Tri-County Supply Canal.

1.2.3.2 Current Conjunctive Management in the CNPPID System

CNPPID has been delivering water for irrigation since the early 1940s. Seepage losses regularly occur in the system of canals and laterals used to distribute surface water. Over the years, the seepage losses have created a groundwater mound under the irrigated area served by CNPPID. Approximately 75% of

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the lands served by CNPPID also have groundwater wells available for irrigation supply. As a result, most of the water users in CNPPID’s service area are already using both surface and groundwater, and conjunctive water management is occurring to a certain extent.

Conjunctive management was specifically considered when CNPPID rehabilitated the E-65 Canal system. The rehabilitated system provides recharge where irrigators partially rely on groundwater, and water is delivered via lined canals/lateral and pipelines where irrigators rely primarily on surface water.

1.2.3.3 Proposed Enhancements to Conjunctive Water Management at CNPPID

The project described in this report would increase the level of intentional conjunctive water management at CNPPID. During drier times when streamflows are not as plentiful, the groundwater mound would be the primary source of irrigation supply for CNPPID irrigators. During wet hydrologic cycles, the mound would be recharged using available and excess streamflow. The system of canals and laterals would be used primarily as recharge facilities rather than facilities for irrigation water delivery. CNPPID irrigators would need groundwater wells so that they could access the groundwater mound and the recharge delivered through the canal system.

The potential benefits of these actions are described below: CNPPID irrigators would not be constrained by surface water delivery schedules or shortages of

surface water supply.

By giving more CNPPID irrigators access to the groundwater mound, surface water deliveries could be curtailed during dry times, and surface water that would have been delivered for irrigation could be left in or returned to the river through the J2 or Jeffrey returns to help meet the goals of the PRRIP and the IMPs.

From an irrigation supply perspective, Lake McConaughy could be operated at a lower elevation if storage in the groundwater mound can be utilized by CNPPID irrigators during dry times. The increased available storage space in Lake McConaughy could then be used to capture and manage more excess streamflow during wet or flooding conditions.

Water levels in Lake McConaughy could be maintained at a more stable level, which would offer benefits to recreational users of the lake and shoreline.

Water could be diverted into the canal and lateral system for recharge purposes when flows are available and not on a schedule dictated by crop demands, leading to more operational flexibility.

CNPPID diverts and delivers large volumes of water on an annual basis. If CNPPID irrigators could rely on the groundwater mound and deliveries of surface water could be curtailed during dry times when the Platte River is below target flows, significant flow improvements could be realized and the goals of the PRRIP and IMPs could be greatly advanced, if not achieved.

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Section 2

Study Objectives and Tasks The overall goal of this study was to understand, from a technical perspective, the conceptual-level feasibility and potential benefits of enhancing conjunctive water management in CNPPID’s service area. For the purposes of the study, the overall goal was broken down into several study objectives. The study was conducted at a conceptual level and sought to provide preliminary answers to some basic questions regarding how CNPPID operations could be altered to enhance conjunctive water management. The study objectives were as follows:

Investigate the conceptual-level feasibility of shifting the operating objectives for the canal and lateral system to providing recharge rather than direct deliveries of surface water for irrigation.

Understand the potential effects of enhanced conjunctive water management on hydropower production with a goal of maintaining or increasing hydropower above current levels.

Estimate potential Platte River flow increases resulting from conjunctive water management that would help Nebraska meet the goals of the PRRIP and get from over-appropriated to fully appropriated in the Platte River basin.

Identify potential recreational and flood control benefits at Lake McConaughy resulting from conjunctive water management.

Estimate non-beneficial consumptive use savings that could be achieved.

To meet the study objectives, the following tasks were conducted. The assumptions used to carry out these tasks, the approach for each task, and the results are described in subsequent sections of the report. Identify lands without irrigation wells. Irrigators will need to be able to access the groundwater

mound and the recharge provided through the system of canals and laterals if direct deliveries of surface water for irrigation no longer occur. In this task, lands without irrigation wells and the number of new wells needed to give CNPPID irrigators access to the groundwater mound were estimated.

Identify areas with inadequate aquifer. Conjunctive water management, in this application, will rely on aquifer storage to supply irrigation needs during dry times. Aquifer properties and other information were reviewed to identify areas within CNPPID’s service area where hydrogeologic conditions would not support conjunctive water management.

Assess the existing delivery system’s ability to recharge. The system of canals and laterals will be an important mechanism for providing recharge in a conjunctive water management program. In this task, historical amounts of seepage, the density of the canal/lateral network, depth to groundwater, and other factors were considered to assess the recharge capabilities of the existing delivery system and the need for additional facilities to enhance recharge.

Develop a water budget. The proposed conjunctive water management program will need to be sustainable in its use of the groundwater mound and in maintaining the mound’s longevity, providing

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adequate supplies of irrigation supply through recharge, etc. A water budget was developed to understand crop water needs, historical conveyance seepage losses, on-farm losses, and other components and to serve as a guide in assessing the sustainability of different water management scenarios.

Assess the effects and benefits of conjunctive water management. Three scenarios of conjunctive water management were developed and conceptually applied to CNPPID’s irrigation delivery system. Surface water and groundwater modeling tools were used to assess the potential effects and benefits of conjunctive water management scenarios to the following factors:

Surface water flows and storage

Fish and Wildlife Service target flows and instream flow rights

Hydropower production

Groundwater levels

Evaporation from Lake McConaughy

Other non-beneficial consumptive use

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Section 3

Assessment of Existing Conditions Several of the study tasks described in Section 2 were developed to investigate the feasibility of using existing facilities and the aquifer in a conjunctive water management program and needed improvements to existing facilities. Section 3 describes the assumptions, approach, and results of these tasks.

3.1 Assessment of Access to the Groundwater Mound Much of CNPPID’s service area has access to groundwater for irrigation. Irrigators with existing groundwater wells already practice some level of conjunctive water management and would readily be able to participate in the conjunctive water management program described in this report. However, CNPPID’s service area includes fields that do not have irrigation wells, and these lands would need new irrigation wells to allow participation in a conjunctive water management program. It should be noted that the assessment assumed that irrigators under the E-67 Canal system would not participate in the conjunctive water management program (see Section 4.2 for further explanation). As a result, lands under the E-67 Canal system were not included in the assessment of access to the groundwater mound.

A conceptual-level assessment was conducted using GIS to estimate the number and size of fields that would need new irrigation wells. From this assessment, the number of new irrigation wells necessary for the conjunctive water management program was estimated.

3.1.1 Approach and Assumptions

The approach for this task relied on maps of several features including irrigated lands, surface water irrigated lands and registered, active irrigation wells. GIS was used to overlay the mapping layers and to identify potential fields that would need irrigation wells. The mapped data used for the assessment is described below:

Irrigated lands: The University of Nebraska’s Center for Advanced Land Management Information Technologies (CALMIT) developed maps of irrigated lands reflecting 2005 conditions for COHYST. The map layers were developed using satellite imagery and other data. Boundaries of irrigated parcels in the CALMIT data sets more closely match the actual shape of irrigated parcels than the maps of surface water irrigated lands described below. Two maps of irrigated lands were developed by CALMIT – one for center pivots and one for “other irrigation” (irrigated lands that do no have center pivots). For the purposes of this study, it was assumed that the 2005 mapping of irrigated parcels is reflective of current conditions.

Surface water irrigated lands: Lands irrigated with surface water were identified and mapped during the development of groundwater modeling tools for COHYST. The map layer shows the location of parcels irrigated with surface water as well as the water supplier and the area of each parcel. Lands

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receiving CNPPID irrigation water are depicted in the maps as squares or rectangular shaped areas that generally correspond to the shape and location of the irrigated parcel.

Irrigation wells: The NDNR maintains databases and GIS maps of registered wells. The GIS maps included data describing the use of the well, its status (i.e. active, inactive, etc.) and other data.

Several analysis steps were taken to estimate the number of wells necessary to provide CNPPID irrigators access to the groundwater mound. The steps are described below: 1. The maps of center pivots and other irrigation from CALMIT were overlaid on the map of surface water

irrigated lands. Parcels of land were then identified that were irrigated with center pivots and other irrigation in 2005 and can also receive surface water from CNPPID. The resulting map layer showed CNPPID surface water irrigated parcels with locations and shapes that were more reflective of actual conditions than the original map coverage of surface water irrigated lands.

2. A map of registered, active irrigation wells was overlaid on the map resulting from step 1 above. Irrigated parcels with and without irrigation wells that receive CNPPID surface water supplies were identified in this step.

3. The number of wells necessary for the conjunctive water management program was estimated using the map developed in step 2 showing lands without irrigation wells that receive CNPPID surface water supplies. The number of wells was estimated using the following assumptions:

Parcels of land less than 40 acres would not need an irrigation well. The mapping process identified numerous, small parcels of land that receive surface water from CNPPID and do not have an irrigation well. For example, center pivot corners were frequently identified initially as small parcels of land that would need an irrigation well. It was assumed that center pivot corners could be irrigated using the irrigation well supplying the center pivot. For other small parcels of land, it was assumed that irrigation water could potentially be supplied from other nearby wells and that it might not be economical to install irrigation wells on very small parcels.

Irrigated lands with center pivots but no groundwater supply would need one well to supply the center pivot regardless of the parcel size.

Lands needing wells that are not irrigated with center pivots and are greater than 40 acres would require one well for every incremental portion of land equal to or less than 80 acres. The following are examples of how this was applied:

A 60-acre irrigated field without groundwater supply would need one irrigation well.

A 100-acre irrigated field without groundwater supply would need two irrigation wells.

A 180-acre irrigated field without groundwater supply would need three irrigation wells.

A 180-acre irrigated field with one existing irrigation well would need two additional irrigation wells.

Some parcels in the map of lands with surface water irrigation supplies did not correspond to irrigated lands in 2005 mapped by CALMIT. It was assumed that these lands were not irrigated in 2005 but that they were irrigated in the past and might be irrigated in the future. Lands in this category without irrigation wells were identified, and the number of irrigation wells needed was estimated using the assumptions described above.

As described previously irrigators that receive their supply from the E-67 system would likely not participate in the conjunctive water management program and would not require irrigation wells. Lack of adequate groundwater supply led to the construction of the E-67 system in 1954, and it was assumed that hydrogeologic conditions in the area around the E-67 system would not support enhanced conjunctive water management.

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The graphic to the right illustrates the analysis process described above. The light green and light blue parcels are 2005 irrigated areas mapped by CALMIT. The gray rectangular parcels are the lands receiving irrigation water from CNPPID that were mapped as part of the COHYST study. The dots represent active registered irrigation wells. Several parcels are highlighted in the graphic and their different colors depict how the parcels would have been categorized in the analysis. The parcels are described below:

The dark green parcel was within the boundary of CNPPID irrigated lands, is irrigated with a center pivot, and has an active irrigation well at its center. This parcel would have been categorized as land that receives irrigation water from CNPPID and has an irrigation well already.

The light blue parcel was within the boundary of CNPPID irrigated lands, is irrigated with a center pivot, but no irrigation well is located on the parcel. This parcel would have been categorized as land that receives irrigation water from CNPPID and needs one irrigation well to access the groundwater mound to supply the center pivot.

The dark blue parcel was within the boundary of CNPPID irrigated lands, is irrigated with something other than a center pivot (i.e. gated pipe or other system), and there are no active irrigation wells located on this parcel. This parcel would have been categorized as needing three irrigation wells (the parcel is greater than 160 acres) to access the groundwater mound for irrigation supply.

The red parcels were within or touching the boundary of CNPPID irrigated lands and irrigation wells are not located on the parcels. Each of the four red parcels is less than 40 acres, and it was assumed that a new irrigation well would not be needed to supply irrigation water to the parcels.

3.1.2 Results

Using the process described in Section 3.1.1, the number of wells needed to provide groundwater to CNPPID irrigators that are wholly dependent on surface water was estimated to be approximately 450 wells. Figure 3-1 shows CNPPID irrigated lands that currently have irrigation wells and lands that need irrigation wells along with the number of wells needed on each parcel. The figure shows that lands needing irrigation wells are fairly evenly distributed throughout CNPPID’s service area.

The assessment also showed that approximately 75% of CNPPID lands already have irrigation wells, which is consistent with recent reports from CNPPID.

It should be noted that this assessment was conducted at a conceptual level and is subject to the following assumptions and limitations: Land ownership was not considered. It is possible that, for example, an 80-acre parcel (as defined by

the CALMIT mapping) needing irrigation wells may be actually be two separate fields owned by two land owners. It is likely that the two land owners would each install one well on their respective fields (i.e. two wells would be needed to irrigate the 80 acres). The process used for the assessment, however, would have suggested one well is needed. Conversely, a land owner with a 90-acre parcel would potentially only need one well to irrigate their land, but the assessment would have shown that two wells are needed.

The assessment assumes that the mapping of 2005 irrigated lands is reflective of current conditions.

Existing groundwater wells can provide adequate irrigation supply to the lands they serve.


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3.2 Hydrogeologic Conditions of the Aquifer The ability of the regional aquifer to yield suitable quantities of water as well as absorb significant recharge in the vicinity of the CNPPID service area was evaluated in a qualitative manner using the existing distribution of groundwater wells and hydraulic parameter distribution from the COHYST groundwater model.

The present day distribution of irrigated land with access to groundwater wells (green polygons on Figure 3-1) suggests that significant groundwater resources are currently being utilized within the CNPPID service area. Additionally, the surrounding agricultural lands within a 5 mile buffer of the CNPPID service area are served by an extensive distribution of groundwater wells. The large number and spatial distribution of groundwater production wells supports the conclusion that the regional aquifer is viable and capable of supporting pumping for irrigation purposes. The only exception to this conclusion is the area in the immediate vicinity of the E-67 canal system, which has historically experienced poor hydrogeologic conditions (CNPPID, 2011).

Likewise, the distribution of hydraulic parameters from the COHYST model suggests that significant groundwater resources are available throughout the CNPPID service area, as well as immediately adjacent to their serviced lands. Calibrated hydraulic conductivity values within and adjacent the CNPPID service area range from 5 to 240 feet/day (Peterson, 2007), values indicative of a productive aquifer system capable of producing adequate water supply for irrigation. Additionally, given the degree of groundwater rise within the vicinity of the CNPPID service area over the past 50 years, it is likely that groundwater reserves within and adjacent to the CNPPID service area have been supported not only by suitable hydraulic aquifer conditions, but also a regular supply of groundwater recharge. This confluence of favorable conditions strongly supports that the hydrogeologic conditions of the Ogallala aquifer are adequate for continued and additional development as a source of irrigation water supply as well as a receptor for future planned recharge and replenishment efforts. In areas closer to the Platte River, depths to the water table are more shallow and although hydraulic conductivity values are generally higher, the small volume of storage potential in the vadose zone will limit recharge potential in these areas and favor recharge activities further from the river. This factor was considered in the recharge strategies listed below in Section 3.3.3.

3.3 Assessment of Infrastructure For the conjunctive management scenarios considered during this study, existing infrastructure was assumed to be in place and usable for groundwater recharge purposes. Estimates regarding the magnitudes and rates of groundwater recharge are detailed below.

3.3.1 Historical Losses from CNPPID Canals and Laterals

A high-level analysis of historical delivery losses from CNPPID canals and laterals was conducted to derive a general understanding of annual and monthly magnitudes of losses.

As a part of the COHYST study, CNPPID compiled records of diversions of water into the irrigation distribution system and deliveries of water to farms. The difference between diversions and deliveries are losses from evaporation and seepage. In general, seepage losses greatly exceed evaporative losses in unlined surface water distribution systems. Historical evaporative losses in the canals and laterals were not specifically estimated for the purposes of this study. As a result, seepage loss estimates presented in this section may be slightly overstated.

Annual estimates of seepage loss and delivery efficiency were developed based on data from CNPPID for the 1954 to 2007 time period. Figure 3-2 shows the results of the estimates and depicts an increasing


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trend in delivery efficiency. Overall system losses averaged approximately 140,000 AF per year from 1954 to 2007, but in recent time periods (after the early 1980s), annual delivery losses have been in the range of 100,000 to 120,000 AF per year. Again, the great majority of these losses are due to seepage. The delivery loss estimates show that the existing canal and lateral system have a large capacity to provide recharge to the aquifer.

Average monthly delivery losses were also estimated. The distribution of monthly delivery losses were proportioned based on the historical distribution of monthly diversions of water into the irrigation system. Historical monthly diversions into the Phelps, E-65, and E-67 systems were obtained from OPSTUDY (OPSTUDY is described in subsequent sections of this report). Monthly estimates of delivery losses are shown in Figure 3-3. The figure shows average monthly losses at their maximum in July and August at approximately 40,000 AF per month.

A more rigorous assessment of monthly losses that incorporates seasonal changes in loss rates may yield slightly different results and may show that monthly loss rates are higher early in the irrigation season as the canal and lateral system is “charging” and lower later in the irrigation season. The high-level analysis conducted was sufficient to meet the purpose of this analysis and provided a general understanding of monthly delivery losses.

3.3.2 Spatial Distribution of Groundwater Recharge from Canals

For the purposes of estimating the impact of recharge from existing canals and infrastructure, a radial influence of 1 mile was assumed for potential hydraulic response, or “mounding”, around open canals and laterals within the CNPPID service area (Figure 3-4). In other words, it was assumed that, over the short term, aquifer levels within 1 mile of a canal or lateral would increase due to seepage when water is being delivered through the canal or lateral. Over the long term, the local aquifer responses translate into more regional aquifer responses. It should be noted that buried laterals were not included in this assessment. Additional modeling and calculations are required to further refine the lateral distance and magnitude of mounding adjacent to existing CNPPID infrastructure; however, this preliminary estimate provides a likely extent of mounding potential relative to existing linear recharge sources; given the magnitude of canal leakage estimates presented in Section 3.3.1. Furthermore, the extent of the assumed 1-mile buffer allows for the consideration of various recharge strategies that would effectively augment groundwater supplies within the CNPPID service area given the current distribution of irrigation infrastructure.

In general, the mapping-based assessment described above supports the conclusion that the network of existing CNPPID canals and laterals are well distributed throughout the CNPPID service area and could provide recharge to most suitable locations. In areas where open canals and laterals do not currently exist, it is possible that additional, engineered recharge facilities may need to be constructed. Additional groundwater modeling would be useful in further assessing the need for and efficacy of additional recharge facilities.

3.3.3 Recharge Strategies

Given the estimates of seepage loss from the existing canals and laterals from the 1954 to the 2007 time period and the assumed radius of influence of hydraulic response, or mounding, from future potential groundwater recharge, three classes of groundwater recharge were identified as potential strategies for future, conjunctive management scenarios (Figure 3-4). These strategies are listed and described below:

Distributed Recharge, which refers to the use of existing canals, laterals, and field application to spread a given volume of groundwater recharge over a large areal extent.


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Localized Recharge, which refers to the use of engineered, rapid infiltration basins and underground injection wells to emplace groundwater recharge in high volumes in localized areas.

Recharge and Recovery, which refers to managed recharge and groundwater extraction using existing canal and lateral infrastructure and current as well as future pumping wells to maintain an optimal depth to groundwater for crops as well as irrigation wells.

As illustrated on Figure 3-4, three distinct areas of recharge strategies have been identified given the assessment criteria of 1) current depths to groundwater and proximity to perennial drainages associated with the Platte River, 2) proximity of existing recharge infrastructure, and 3) future, estimated mounding potential associated with existing canals and laterals. However, these recharge strategies may be applied where appropriate throughout the CNPPID service area and adjacent region depending upon land availability, localized hydrogeologic conditions, and groundwater demands.

Along the E-65 canal, within the western third of the CNPPID service area, where depths to groundwater and associated, underground water storage potential are greater than those immediately adjacent to the Platte River, distributed and localized recharge are both viable options for conjunctive water management.

Immediately south of the central portion of the CNPPID service area, between the E-65 and eastern extent of the Phelps canal is an area that may be suitable for higher volume, localized groundwater recharge operations. This area has greater depths to groundwater and less influence from existing canal and lateral recharge infrastructure and has a greater distance from the Platte River, suggesting that this region would be a suitable area to store recharged groundwater that would augment aquifer reserves in both the CNPPID service area as well as the portion of aquifer extending southwards to the Republican River basin.

The eastern portion of the CNPPID service area served by the Phelps canal is suitable for localized groundwater recharge and recovery operations. Aquifer conditions in this area are characterized by shallower depths to the water table and are more susceptible to negative impacts to agricultural fields from high water tables as well as seepage into subsurface domestic structures. A managed recharge and extraction regime in this area would not only help optimize conjunctive water management, but could also improve waterlogged conditions (to the extent that they exist) in the eastern extent of the CNPPID service area via managed groundwater extraction and management of seasonal water table elevations. This beneficial use of the current and future groundwater mound would not only provide additional water for irrigation, but also reduce the potential for future property damage from high water tables, associated seepage, and flooding.

Although no optimization procedures were performed to estimate a more precise distribution of appropriate recharge strategies for the various regions of the CNPPID service area, all three recharge strategies would be appropriate for application within portions of the study area and all may be beneficial for both water supply as well as protection of property from seepage and damage from periodic, elevated water tables. Given these qualitative conclusions, additional quantitative, modeling analyses would be useful for estimating the relative beneficial impact of distributed recharge options in a series of “bookend” scenarios, which reflect end member groundwater recharge options as discussed in Section 4.3. Optimization of these various recharge scenarios, given local hydrogeologic conditions and land use patterns, will likely improve upon the volume of available water supply for river flow and irrigation as well as increase the reliability of agricultural water supplies. However, such optimization analyses were not performed as part of this conceptual-level study.


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3.4 Water Budget Development A water budget was developed for CNPPID’s system as a guideline for creating and evaluating conjunctive water management scenarios. The water budget also served as a useful tool for understanding the relative magnitude of water volumes diverted, returned, seeped, and consumed.

Data for the water budget was obtained from NDNR gaging records, CNPPID records of diversions into the irrigation system and on-farm deliveries, and estimates of net irrigation requirements (NIR) derived using output from the CROPSIM model.

3.4.1 Process for Estimating Net Irrigation Requirement

Estimates of crop consumptive use, NIR, and other on-farm water budget components were based on existing CROPSIM model output developed for the COHYST modeling project. CROPSIM used climate data from weather stations across the COHYST modeling area to calculate consumptive use, NIR, etc. for various types of crops and vegetation growing on various types of soils. For development of the water budget, output from the CROPSIM model was applied to lands receiving surface water supplies from CNPPID to estimate the NIR in CNPPID’s service area.

CROPSIM output using climatic data from the Gothenburg, Holdrege, Medicine Creek Dam, Minden, and North Platte WSO ARP weather stations was used in the estimate of NIR for CNPPID’s service area. CROPSIM output for the various weather stations was applied to different parts of CNPPID’s service area depending on the location of each field relative to the nearest weather station.

Cropping patterns in CNPPID’s service area were obtained from GIS-based land use mapping developed by CALMIT reflecting 2005 conditions. The 2005 CALMIT land use mapping covers the entire state of Nebraska and describes land uses based on 25 categories that include a variety of irrigated and dryland crops, grasslands, urban areas, open water, etc. The 2005 CALMIT land use mapping was assumed to represent current land use conditions.

GIS was used to combine CALMIT mapping of 2005 land use, weather station locations, CNPPID irrigated fields, and maps of soil properties. Each of these mapping coverages were intersected in GIS, resulting in a mapping coverage that included polygons of fields in CNPPID’s service area with attributes describing specific soil types, land uses, and coverage of weather stations. For each of these polygons, annual NIR amounts could be obtained in the CROPSIM output based on the soil type, land use, and weather station used by CROPSIM. The extraction of CROPSIM output for each individual polygon was conducted using queries of GIS and CROPSIM output databases. Annual NIR based on 2005 land uses was estimated by multiplying annual NIR amounts for each polygon within CNPPID’s service area (in terms of inches per acre) by the land area for each polygon, resulting in an annual NIR volume for each polygon. The NIR volumes calculated for each polygon were summed on an annual basis for all of the polygons receiving CNPPID irrigation water. The annual NIR volumes calculated using this process represent NIR relative to 2005 land uses projected across historical climatic records.

3.4.2 Water Budget Results

Data describing the components of the water budget were compiled from the sources described above and were summarized based on annual averages. The data sources had varying time ranges over which data were available. Components of the water budget were averaged beginning in 1954 (the first year of deliveries under the E-67 system) and ending in the last year for which data was obtained. Table 3-1 summarizes the water budget and shows the data source and timeframe over which averages were calculated for each water budget component. Water budget data were estimated on a system-wide basis and for the irrigation system.


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Table 3-1. Water Budget for CNPPID System

Component Amount (AF/year)

Data Source Timeframe

Syst

em-W

ide

Total Annual Diversions into the Tri-County Supply Canal

1,074,000 NDNR gage 142000 1954-2008

Returns through Jeffrey 47,000 NDNR gage 143000 1954-2008

Returns through J-2 547,000 NDNR gage 144000 1954-2008

Seepage and Lake Evaporation Prior to Delivery to Irrigation System

257,000 Calculated remainder


Irrig

atio

n Sy

stem


Seepage Loss in Canals and Laterals 138,000 CNPPID records 1954-2007

Historical CNPPID On-farm Irrigation Deliveries

97,000 CNPPID records 1954-2007

Net Irrigation Requirement 71,000 Processed CROPSIM output

1954-2005

The data in Table 3-1 show the following: Approximately 55% of the water diverted at the Tri-County Supply Canal dam is returned to the Platte

River through the Jeffrey and J-2 Returns. The water provided to the system of canals and laterals (235,000 AF/yr) is approximately 22% of

diversions into the Tri-County Supply Canal.

The efficiency of the irrigation delivery system (on-farm deliveries divided by the amount provided to the irrigation system) is approximately 41% based on long term averages. However, the efficiency has improved over time as shown in Figure 3-2.

The irrigated area under CNPPID is approximately 112,000 acres, but it has fluctuated somewhat over the years (the CNPPID website mentions that irrigated acreage grew to 123,000 acres at one point in its history). Water budget components reduced to a per-acre basis (assuming 112,000 acres) are shown below. Because historical acreage has fluctuated, the per-acre estimates are not exact, but they should be generally reflective of actual amounts.

Amount provided to the irrigation system: 25 inches/acre

Historical CNPPID on-farm irrigation deliveries: 10 inches/acre

Net irrigation requirement: 8 inches/acre

On-farm irrigation technologies in the CNPPID service area include a mixture of center pivots and furrow-based systems (i.e. gated pipe, etc.). Overall application efficiency for furrow based systems typically ranges from 50 to 90 percent, and center pivots average 75 to 95 percent (Howell, 2003). Actual on-farm efficiencies depend on soil type, topography, irrigation amounts, specific types of irrigation equipment, etc. Actual application efficiencies for CNPPID farms were not estimated as a part of this study. For illustrative purposes, assuming a 70 percent overall application efficiency (which accounts for both furrow-based and center pivot irrigation systems), the amount of water provided to crops for consumption would average approximately 68,000 AF/yr based on annual on-farm deliveries of 97,000 AF. Water that was delivered to the farm but was not available to crops ran off the end of fields, percolated below the root zone, or was lost via other system inefficiencies. The


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amount of water provided to crops for consumption (68,000 AF/yr) is very similar to, but slightly less than the net irrigation requirement shown in Table 3-1 estimated using CROPSIM output.

The following is a summary of the “fate” of water that is diverted into the CNPPID system at the Tri-County Supply Canal Dam. Figure 3-5 illustrates the information below.

Approximately 55% of diversions are returned to the Platte River through the Jeffrey and J2 returns.

Approximately 24% of diversions seep into the aquifer or evaporate prior to reaching the irrigation canals and laterals.

Approximately 15% of diversions seep into the aquifer or evaporate while being conveyed in irrigation canals/laterals or is not consumed on-farm because of irrigation inefficiencies.

Approximately 6% of diversions are consumed by crops.

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Section 4

Assessment of Effects and Benefits of Conjunctive Water Management A conceptual-level assessment of the potential effects and benefits of various conjunctive water management scenarios was conducted using available surface and groundwater modeling tools. The assessment relied on existing modeling tools that have previously been applied in the central Platte River valley to evaluate various surface water and groundwater related issues and programs. The surface water and groundwater modeling tools used for the study were not directly linked (i.e. changes in surface water management are not automatically input and analyzed by a groundwater modeling tool).

An integrated surface water and groundwater modeling tool is currently being developed as a part of the COHYST project. The integrated model will use CROPSIM, MODFLOW (as applied in the COHYST groundwater modeling project), and Stella to dynamically assess the entire water budget in evaluating conjunctive water management scenarios in the central Platte River valley. Once developed, this tool would likely be ideal for evaluating the conjunctive water management program described in this report. However, the model was not available as of the date of this report; thus, a series of peer reviewed and accepted modeling tools were used to estimate the benefits and relative effects of various conjunctive water management practices.

4.1 Modeling Tools Individual surface water and groundwater modeling tools were used to assess the effects and benefits of the conjunctive water management program described in this report. The surface water modeling was conducted using OPSTUDY, and the groundwater modeling was done using the Eastern Model Unit of the COHYST groundwater model (a MODFLOW flow model). The models, modeling assumptions, and modeling limitations are described in the following sections.

4.1.1 OPSTUDY

OPSTUDY is a surface water modeling software package developed by the U.S. Bureau of Reclamation. It has been applied in the central Platte River valley on several occasions to evaluate water management programs and their effects on Lake McConaughy and flows in the Platte River. The model has been modified several times in the past to evaluate different water management scenarios.

OPSTUDY functions as a surface water accounting model that can be used to evaluate diversions and returns to the Platte River system and account for gains, losses, and stream flows. The model is bounded on the upstream side at Lewellen, Nebraska on the North Platte River and Julesburg, Colorado on the South Platte River. The downstream boundary is Duncan, Nebraska. Central Platte River valley surface water management protocols and facilities are included in the OPSTUDY model, which allows the

Section 4 Assessment of Effects and Benefits of Conjunctive Water Management

4-2

user to evaluate river flows, irrigation diversions, hydropower generation, reservoir storage amounts and releases, etc. The model accounts for stream flow gains in the Platte River using historical, static estimates. However, because OPTUDY is a surface water model, it cannot dynamically modify stream flow gains based on groundwater management activities. For the purposes of this study, OPSTUDY was run on a monthly time step.

4.1.1.1 Modifications to OPSTUDY

The version of OPSTUDY used for this study was based on Central Platte River OPSTUDY model (CPR Model) developed by the Fish and Wildlife Service for the Environmental Impact Statement (EIS) concerning the PRRIP. The CPR Model was used during the EIS study to evaluate water management alternatives and their effects on stream flows and diversions overlaid on 1947 through 1994 climatic conditions. The modeling alternatives were compared to a baseline modeling run or “Present Conditions” model that was intended to reflect pre-PRRIP operating criteria and demands in the central Platte River system simulated using 1947 to 1994 hydrologic conditions (PRRIP, 2006). The Present Conditions version of the CPR Model was used for the purposes of this study.

The modeling time period of the Present Conditions model was altered to reflect 1952 to 2002 conditions. Model and resource limitations prevented the expansion of the modeling time period beyond 50 years. The year 1952 was chosen as the starting point for the model to capture the drought of the 1950s. The drought of the early 2000s was not captured in the modeling efforts for this study due to model and resource limitations, but Brown and Caldwell recommends that future efforts extend the modeling time period to present day so that the effects of the drought on conjunctive water management scenarios can be evaluated. In addition, because the modeling period did not capture the full cycle of the early 2000s drought, modeling results were evaluated over the 1952 to 2001 timeframe (2002 was the first year of the early 2000s drought).

To extend time period of the Present Conditions model, monthly diversion demands, reach gains, and stream inflows were obtained from a version of OPSTUDY representing historical conditions from 1931 to 2002. The historical data were used to extend the Present Conditions model to include the years 1995 through 2002. The extended Present Conditions model served as a baseline model run against which conjunctive water management scenarios were evaluated. To avoid confusion with the CPR “Present Conditions” Model, the baseline model used for this study will hereinafter be referred to as the Baseline Conditions model.

4.1.1.2 Description of OPSTUDY Modeling Process

Because resources were limited for this study and it was conducted at a conceptual level, OPSTUDY was used in way that did not require significant changes to the model but that would approximate the effects and benefits of conjunctive water management. The OPSTUDY modeling framework consists of model code for conducting water accounting, estimating hydropower production, etc. and input data files that provide monthly diversion demands, river gains, and stream inflows. The OPSTUDY model code was not altered for this study. Instead, diversion demands in the data input files were changed to simulate conjunctive management scenarios.

CNPPID irrigation demands and Kearney Canal hydropower demands were the primary inputs that were modified to simulate conjunctive water management. The demands were increased or decreased based on stream flows at Grand Island and storage in Lake McConaughy. CNPPID irrigation demands were based on the provision of water for recharge and other uses. Several assumptions were made in developing CNPPID diversion demands for recharge, and they are described in Section 4.2 of this report. Kearney Canal hydropower demands are non-consumptive and were used to draw releases from Lake McConaughy. The following describes how these demands were used to simulate conjunctive water management during dry, normal, and wet hydrologic cycles:

Assessment of Effects and Benefits of Conjunctive Water Management Section 4

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Dry: During drier hydrologic periods, CNPPID irrigation demands were partially or greatly reduced to either maintain storage in Lake McConaughy or provide additional stream flow at Grand Island. During drier hydrologic cycles, the groundwater mound was the primary source of water supply to CNPPID irrigators. Kearney Canal hydropower demands were increased to draw some or all of the savings from reduced irrigation demands or accumulated storage out of Lake McConaughy to boost stream flow.

Normal: During “normal” years, CNPPID irrigation demands were based on yearly crop consumptive use and canal/lateral losses. In “normal” years, the objective of recharge operations was to simply maintain the groundwater mound. If stream flows at Grand Island were below FWS targets, Kearney Canal hydropower demands may have been increased to draw storage from Lake McConaughy to increase stream flow at Grand Island. However, storage releases may have been limited if storage levels in Lake McConaughy were low.

Wet: During wet hydrologic cycles, irrigation demands were increased to use excess stream flow to recharge and build up the groundwater mound.

Modeling of conjunctive water management scenarios was conducted by making multiple OPSTUDY runs. In any particular year, storage amounts in Lake McConaughy depend on releases and storage in previous years. Because of this, successive model runs focused on balancing Lake McConaughy storage, diversions to recharge, and boosting stream flows in earlier time periods with the focus progressing to later time periods with each model run. In each successive model run, CNPPID diversion demands and Kearney Canal hydropower demands were altered based on recharge needs, storage conditions in Lake McConaughy, and stream flow shortages.

4.1.2 COHYST

The COHYST study began in 1998 with the objective of developing hydrologic databases, analyses, models, and other information that would assist Nebraska in the PRRIP and in developing water policy. COHYST will provide tools to better understand surface and groundwater resources and connections and the effects of water management activities.

The COHYST modeling area stretches from the Republican River/Frenchman Creek on the south to the Loup and South Loup Rivers and a groundwater divide on the north. In the east, the COHYST modeling area ends near Columbus, Nebraska and the western boundary roughly corresponds to the Colorado and Wyoming statelines.

Groundwater flow modeling in COHYST is conducted with MODFLOW. Often the term “COHYST model” refers to the MODFLOW groundwater model developed during the COHYST study. The term “COHYST model” will be used in this report to refer to the Eastern Model Unit of the Nebraska COHYST groundwater model, which encompasses the entirety of the project study area.

The COHYST model was developed and calibrated over a period of several years, and it has been used for numerous groundwater analyses, assessments of groundwater pumping and depletions to surface flow, etc. The hydrologic databases developed through the COHYST study have been useful tools in evaluating hydrologic issues in the Platte River valley.

4.1.2.1 Integration of groundwater modeling.

The Eastern Model Unit of the COHYST model encompasses the study area as well as the whole of the CNPPID service area and was used as the groundwater model framework for assessing changes in aquifer reserves given the conjunctive management water scenarios discussed below. Additionally, consumptive water demands, river diversions, and availability of water for groundwater recharge determined from the OPSTUDY simulations were directly applied to the groundwater inflow and outflow stresses for the Eastern Model Unit of the COHYST model. Pumping within the COHYST model was


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based upon net irrigation requirements for lands within the CNPPID service area under the E-65 and Phelps Canals using crop water demand estimates from CROPSIM modeling (the pumping estimates assume that all irrigators under these canals will use groundwater recharge as their source of irrigation supply and that irrigation needs will be fully satisfied). Groundwater recharge was dependent on the surface water supplies available for diversion within a given year based upon Platte River flows, Lake McConaughy storage, and model solutions provided by OPSTUDY (see discussion in Section 4.1.1.2). Pumping and recharge amounts were input into COHYST based on annual stress periods. For all modeled conjunctive use scenarios groundwater pumping was evenly distributed throughout the CNPPID service area (excluding the E-67 area) at a total magnitude suitable for meeting the full consumptive crop requirement for the CNPPID service area. Groundwater recharge was applied to groundwater model cells that contained the E-65 and Phelps canals and laterals. For model scenarios where recharge was applied along both the E-65 and Phelps canal systems; approximately two-thirds of the total volume of available annual recharge was applied along the E-65 canals and laterals, and approximately half that total volume was applied along the total length of the Phelps canal system. This division of recharge was selected to generally emplace more stored groundwater within the E-65 system, which generally exhibits greater depths to groundwater and greater subsurface storage capacity with the present day vadose zone.

4.2 Assumptions Several assumptions were used in the creation of conjunctive water management scenarios and the development of modeling inputs. The assumptions and their effects to modeling inputs are described below:

The E-67 system would not be included in a conjunctive water management program. The E-67 system was developed in the early 1950s because area irrigators had trouble accessing adequate groundwater supplies. Hydrogeologic conditions in the E-67 service area would potentially not be conducive to conjunctive water management and irrigators may prefer to rely on surface water. In addition CNPPID recently conducted an improvement project on the E-67 system to line canals and replace open laterals with buried pipe. For these reasons, the OPSTUDY model for each scenario included historical diversion demands based on direct delivery of surface water for irrigation in the E-67 system.

Improved irrigation efficiency will result in less non-beneficial consumption. Many irrigators who construct new wells on lands irrigated exclusively with surface water would likely install center pivots. Center pivots are generally more efficient than furrow-based irrigation systems. Non-beneficial consumption from standing water in roadway ditches resulting from end-of-field runoff may be reduced by using more efficient irrigation methods. In addition, water levels in the groundwater mound could potentially be lowered in a managed way, and direct consumption of groundwater by phreatophyte vegetation could be reduced.

Recharge diversions will cover canal/lateral seepage and on-farm net irrigation requirements. Under current operations, diversions into the irrigation system need to cover canal/lateral seepage, consumptive use demands of crops, and on-farm irrigation inefficiencies. The CNPPID diversion demands for recharge used in OPSTUDY do not include water needed to cover on-farm irrigation inefficiencies or the seepage requirements to deliver that water to farms. Rather, CNPPID diversion demands for recharge include on-farm consumptive use requirements and additional water to cover the seepage in canals and laterals that will occur along with the delivery of recharge to meet crop requirements. The majority of on-farm losses from irrigation inefficiencies eventually return to the Platte River via aquifer recharge from deep percolation or surface flow from end-of-field runoff. However, some of the losses likely do not return to the river because of evaporation or consumption by phreatophytes. Platte River gains in OPSTUDY were adjusted downward because diversions for


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recharge did not include amounts for on-farm irrigation inefficiencies that would have resulted in return flows to the Platte River. The gains were reduced by 75% of the on-farm irrigation inefficiency to account for the reduction in return flows. It was assumed that the remaining 25% of reduced delivery would not impact gains because some on-farm inefficiencies do not result in return flows at the Platte River as described above.

Historical contributions to the Republican River basin will be maintained. The groundwater mound under CNPPID’s service extends into the Republican River basin and creates baseflows that help Nebraska maintain compliance with the Republican River Compact. The OPSTUDY model in each conjunctive water management scenario included a recharge demand of 12,000 acre-feet per year that could be delivered to recharge facilities located where water would accrete to the Republican River basin.

Maintenance flows may be necessary. If diversions to recharge are curtailed for several years, some diversion into the canal and lateral system may be beneficial for elimination of weeds and other maintenance issues. The OPSTUDY modeling runs included 30,000 AF of springtime diversions into the canal and lateral system every third year for maintenance purposes if diversions were curtailed for three or more consecutive years.

The value of additional hydropower would be between $0.026 and $0.040 per kWh. CNPPID sells the hydropower it generates to NPPD. NPPD then sells the energy from CNPPID and others to a substantial number of cities and agencies in Nebraska. NPPD’s rate schedule for the sale of energy has different energy rates for the summer and winter seasons and different rates for peak and off times during the day. NPPD solicits renewable power for their system. Unlike most utilities, NPPD does not give a value to the renewable power they are soliciting, but instead asks for entities interested in providing power to make a proposal to NPPD. NPPD does, however, give a range of what it expects the average price of energy to be in 2012. NPPD expects the price of energy to vary between $0.026 and $0.029/kWh (NPPD, 2012). (NPPD numbers were given in $/MWH, but they were converted to $/kWh as that is more commonly used unit used to discuss the value of energy on this scale). In 2010, NPPD also stated that they will not consider wind power with a value that exceeds $0.040/kWh (NPPD, 2010). A floor of $0.026/kWh and a ceiling of $0.040/kWh was assumed to be the price that NPPD would be willing to pay for power from CNPPID. It was also assumed that NPPD would buy power from CNPPID at or nearly at the same rate they would pay for a new resource.

4.3 Conjunctive Water Management Scenarios 4.3.1 Operational Goals of Scenarios

Each of the conjunctive water management scenarios developed for this study had several overall operational goals. The goals were developed as a means to evaluate the modeling results of each scenario. The operational goals are listed in Table 4-2.


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Table 4-2. Operational Goals for Conjunctive Water Management Scenarios

Goal Description

No negative impacts to CNPPID water users A primary goal of the proposed conjunctive water management program is to provide benefits CNPPID water users in terms of increased reliability of irrigation water supplies through management of the groundwater mound and Lake McConaughy.

Reduce shortages of stream flow at Grand Island Through retiming of diversions and using both surface water and groundwater storage, the conjunctive water management program should provide additional stream flow in the central Platte River during times of stream flow shortage.

Beneficially use stored groundwater in the mound By relying exclusively on groundwater during drier hydrologic cycles, the groundwater mound will be used directly by CNPPID irrigators and indirectly as a means to provide additional stream flows. In areas with high groundwater tables, lowering the level of the groundwater mound could benefit agricultural producers.

Alter operation of Lake McConaughy for additional benefits

Each of the modeling scenarios sought to maintain storage amounts in Lake McConaughy between 800,000 AF and 1,200,000 AF. Currently, Lake McConaughy storage levels are generally managed higher than this. The goal of storing less water in Lake McConaughy could result in the following:

• Enhanced recreational opportunities by maintaining shoreline access and stable water levels.

• Additional flood control by preserving reservoir storage space to capture high flows.

• Reduced evaporation losses due to decreased surface area of the lake.

Maintain or increase hydropower production Revenues from hydropower production are critical and should be either maintained or increased by the conjunctive water management program.

Additional benefits to NPPD NPPD has the first right to store inflows into Lake McConaughy at the beginning of the storage season. Once NPPD’s storage amount of 125,000 AF has been filled, CNPPID and the Environmental Account (a storage account held by the U.S. Fish and Wildlife Service) can fill. The conjunctive water management program proposed in this study does not seek to alter NPPD’s storage right in any way. Through more reliance on the groundwater mound and the potential to avoid large declines in storage levels in Lake McConaughy, the conjunctive water management program could improve the supply of cool water for Gerald Gentleman Station and enhance the reliability of supply to the Kearney Canal.

4.3.2 Scenario Descriptions

Three scenarios of conjunctive water management were developed and were evaluated using the modeling tools employed for this study. Scenarios 1 and 2 were developed as “bookends” to examine effects and benefits of conjunctive water management using nearly the entire irrigation distribution system (Scenario 1) and only a part of it (Scenario 2). Scenario 3 incorporated potential effects and benefits of additional storage upstream of Grand Island. Descriptions of the scenarios are provided in Table 4-3.


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Table 4-3. Description of Conjunctive Water Management Scenarios

Scenario Description Sc

enar

io 1

E-65 and Phelps systems in service

The first scenario assumed that recharge deliveries would be distributed using the E-65 and Phelps Canals and associated laterals. The scenario assumed that no deliveries for direct irrigation would be provided in these systems.

Scen

ario

2

Only the E-65 system is used The second scenario assumed that the E-65 Canal and associated laterals would be used to distribute recharge in part of the CNPPID service area. It was assumed that the Phelps Canal would not be needed for distribution of recharge or direct irrigation deliveries, because the water table is relatively high in the eastern portion of the CNPPID service area. CNPPID irrigators under the Phelps Canal would rely on groundwater wells for irrigation supply. Under the E-65 system, groundwater levels are generally lower, and there are more opportunities to recharge. Stream flow gains were reduced to account for the lack of delivery into the Phelps Canal system.

Scen

ario

3

An additional storage facility is constructed upstream of Grand Island

The third scenario used the results of Scenario 1 to assess additional stream flow benefits that could result from the addition of a new surface water storage facility upstream of Grand Island. The purpose of the storage facility would be to capture additional excess stream flow and to fine tune management of shortages and excess. The storage facility could be an off-channel reservoir, an aquifer storage and recovery facility, an existing structure such as Elwood Reservoir (with new infrastructure), etc. The specific location of the storage facility was not specified, and the analysis assumed that the facility would be a surface water reservoir. The evaluation of Scenario 3 was conducted using a spreadsheet tool rather than OPSTUDY and the COHYST model.

4.3.3 Scenario 1 Simulation Results

Scenario 1 was simulated as described in Section 4.1.1.2 by focusing on the recharge diversions, Lake McConaughy storage, and stream flow improvements in early years of the simulation and shifting focus to later years with each successive model run using the assumptions and goals described above. OPSTUDY was used to simulate surface water effects and benefits. The resulting array of recharge demands was input into the COHYST model to evaluate the potential for mounding or decline.

4.3.3.1 Surface Water and Hydropower Effects

Several output parameters from OPSTUDY were examined to assess the results of Scenario 1 with respect to surface water and hydropower effects. The primary output parameters from OPSTUDY used to evaluate Scenario 1 included storage in Lake McConaughy, average Platte River flows at Grand Island, Platte River flows at Grand Island during times of shortage, and hydropower production at Kingsley Dam, the North Platte Hydro, and the CNPPID hydropower facilities. The output parameters for Scenario 1 were compared with the corresponding OPSTUDY output for Baseline Conditions to assess simulated changes in those parameters under conjunctive water management.

Figure 4-1 illustrates how Lake McConaughy and the groundwater mound were used during the simulation of Scenario 1. The bottom half of Figure 4-1 shows a bar chart representing annual average storage in Lake McConaughy in the Baseline Conditions model (blue bars) and as operated under Scenario 1 (red bars). The top half of the figure illustrates how the groundwater mound was used. Negative bars (in orange) represent annual demands for recharge that were not diverted but instead stored in Lake McConaughy or used to boost stream flow. In other words, the orange bars represent water that was “borrowed” from the groundwater mound. The green bars represent annual diversions to recharge and replenish the groundwater mound. Figure 4-1 shows water being borrowed from the groundwater mound during drier periods (i.e. during the 1950s drought and during drier periods in the early/mid 1960s, mid/late 1970s, and early 1990s). During wet periods, Figure 4-1 shows recharge of


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the groundwater mound occurring during wet periods (i.e. the early 1970s, early 1980s and in other years). In years where the figure shows neither an orange nor green bar, diversions were made to recharge in accordance with that year’s crop consumptive demand and seepage requirements (i.e. the mound was maintained).

Scenario 1 effects on stream flows at Grand Island are shown in Figure 4-2. Figure 4-2 shows annual flows at Grand Island in the Baseline Conditions model and under Scenario 1. It also shows FWS annual target flows. During years when flows are below FWS targets, Figure 4-2 illustrates how stream flows were increased at Grand Island in Scenario 1. During high flow years in the Baseline Conditions model, stream flow at Grand Island decreased under Scenario 1, primarily due to recharge diversions into the CNPPID service area.

The results of Scenario 1 simulations with respect to stream flows and hydropower are described in Table 4-4. Further description and explanation of the parameters and output is provided in the discussion following Table 4-4.

Table 4-4. Summary of Scenario 1 Stream Flow and Hydropower Effects

Parameter Change under Conjunctive Water Management Program

Stream Flow






Hydropower Output

Kingsley -9.7 MKWH/yr (decrease)

North Platte Hydro +1.7 MKWH/yr (increase)

Central Supply Canal (Jeffrey, J1, J2) +8.1 MKWH/yr (increase)

Total +0.1 MKWH/yr (slight increase)

Average Annual Platte River Flow at Grand Island. Flows at Grand Island increased by 31,000 AF/yr

on average under Scenario 1. With the inclusion of the Environmental Account, average annual stream flows at Grand Island increased to 46,000 AF/yr. The effects of the Environmental Account were estimated by running Scenario 1 with and without the Environmental Account active. The increases in stream flow can be attributed to less overall diversion demand for irrigation/recharge, less evaporative loss from Lake McConaughy, and operation of Lake McConaughy at a lower average elevation than under the Baseline Conditions model. The components of the average annual increase were estimated as follows:

Environmental Account: 15,000 AF/yr

Additional stream flow from reduced demand for irrigation/recharge diversions: 15,100 AF/yr

Lake McConaughy evaporation savings: 9,000 AF/yr

Additional stream flow from reduced McConaughy storage: 6,600 AF/yr

Average Annual Platte River Flow at Grand Island during Times of Shortage. Under Scenario 1, flows during times of shortage increased at Grand Island by 115,000 AF/yr. The Environmental Account has been estimated to boost stream flows during times of shortage by 50,000 AF/yr on average. The


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combined effects of Scenario 1 and the Environmental Account could increase stream flow during times of shortage at Grand Island by a total of 165,000 AF/yr. Times of shortage were defined as months when the Baseline Conditions model showed flows at Grand Island were below FWS target flows. Figure 4-3 illustrates how flows at Grand Island improved under Scenario 1. The figure shows annual flows at Grand Island during shortage under Baseline Conditions and flows at Grand Island under Scenario 1 during shortage conditions. The annual flows under shortage conditions were determined by summing flows at Grand Island during months when FWS target flows were not being met under Baseline Conditions. Shortage condition flows under Scenario 1 were estimated by summing the Scenario 1 flows at Grand Island during those same months. Figure 4-3 shows flow improvements at Grand Island during times of shortage, which provides a key benefit to the PRRIP. The figure also shows very little improvement when shortages are low (i.e. stream flows are high). It should be noted that the simulated increase in stream flow at Grand Island resulting from Scenario 1 and the Environmental Account during times of shortage exceeds the PRRIP’s first increment goal of stream flow shortage reductions.

Hydropower Output. Under Scenario 1, Lake McConaughy was simulated to operate at a lower level, which resulted in less hydropower output at Kingsley Dam. However, more water was passed down the river and into the North Platte Hydro and CNPPID hydropower facilities. Under Scenario 1 model simulations, the reductions in Kingsley Dam power production were offset by increases in hydropower production at other facilities. In summary, average hydropower production under Scenario 1 did not decrease as compared to Baseline Conditions, but showed a small increase.

Effects to monthly average stream flow at Grand Island under Scenario 1 were also examined. Figure 4-4 shows that average monthly flows at Grand Island either increased or were similar to Baseline Conditions in Scenario 1.

4.3.3.2 Groundwater Effects

Recharge amounts and crop consumptive use requirements (or net pumping demands) from the OPSTUDY modeling were input to the COHYST model to evaluate the potential for long term water table mounding or declines resulting from recharge and pumping operations prescribed in Scenario 1. Crop consumptive use requirements averaged just under 70,000 acre-feet per year, and annual recharge amounts ranged from 100,000 to 400,000 acre-feet depending on available stream flows and recharge needs. Total simulated recharge amounts from canal/lateral seepage in Scenario 1 were similar to (approximately 95%) total historical cumulative canal/lateral seepage amounts.

As previously mentioned, the potential for long term mounding and groundwater table decline was evaluated on a relative, and not an absolute basis. Factors such as variations in groundwater pumping from wells outside of CNPPID or groundwater inflows or outflows from creeks and drains that would intercept groundwater were ignored for this simplified analysis.

Figure 4-5 shows the mounding potential at the end of the 50-year modeling period. After 50 years of pumping to meet the full crop consumptive use demands as well as recharging excess water to the groundwater mound, results from the COHYST model show no regions of groundwater declines within the CNPPID service area over the 50-year simulation time period. Results also suggest that groundwater elevations would have up to 30 feet of mounding potential relative to pre-irrigation conditions. As shown in the figure, no areas within the Phelps Canal and E-65 systems were projected to experience long term water table declines when water was recharged evenly throughout both major canal systems and their laterals.

As groundwater effects were evaluated on a relative or “superposition” basis based upon previous climatic and crop consumptive requirements, it is not expected that the groundwater mound will rise the


4-10

full 30 feet if Scenario 1 were implemented in the future. Groundwater returns to creeks and drains, and consumption of groundwater from non-CNPPID lands would prevent the projected groundwater level rises shown in Figure 4-5; however, it is estimated that there will be a net positive return flow to the regional Ogallala aquifer system under this conjunctive water use scenario.


The approach for simulating Scenario 2 was the same as the approach for Scenario 1. Output parameters from OPSTUDY and the COHYST model were assessed in the same way as described for Scenario 1 above. The results of the Scenario 2 simulations are described below.

4.3.4.1 Surface Water and Hydropower Effects

Figure 4-6 presents Lake McConaughy operations and management of the groundwater mound in the same way as Figure 4-1 (see Section 4.3.3.1). Patterns of groundwater mound management exhibited in Scenario 2 were similar to Scenario 1. During dry periods, water was “borrowed” from the groundwater mound and replaced/recharged during wetter periods. The difference between the two scenarios is in the magnitude of borrowing and replacement. In Scenario 2, the E-65 Canal and associated laterals were the only part of the CNPPID irrigation system assumed to be participating in the conjunctive water management program. As a result, the ability to manage the groundwater mound was more limited than in Scenario 1. The Phelps Canal was not used in Scenario 2, and diversion demands were overall much lower. The lower diversion demands resulted in more available water for storage in Lake McConaughy or for release to boost stream flows. In Scenario 2, average storage in Lake McConaughy was slightly higher than in Scenario 1. The higher average storage was not intentional and was due to the manual nature of the modeling process.

Figure 4-7 shows the effects of Scenario 2 on stream flows at Grand Island. In general, stream flows at Grand Island increased during times of shortage and decreased during times of excess in Scenario 2 (similar to Scenario 1)

The results of Scenario 2 simulations with respect to stream flows and hydropower are described in Table 4-5. Further description and explanation of the parameters and output is provided in the discussion following the table.

Table 4-5. Summary of Scenario 2 Stream Flow and Hydropower Effects

Parameter Change under Conjunctive Water Management Program

Stream Flow






Hydropower Output

Kingsley -6.5 MKWH/yr (decrease)

North Platte Hydro +4.2 MKWH/yr (increase)

Central Supply Canal (Jeffrey, J1, J2) +15.7 MKWH/yr (increase)

Total +13.4 MKWH/yr (slight increase)


4-11

Average Annual Platte River Flow at Grand Island. Flows at Grand Island increased by an average of 64,000 AF/yr in Scenario 2, which is approximately twice the impact of Scenario 1. The increase in average annual flow was primarily due to the lower diversion demand into the CNPPID system. The components of the average annual increase were estimated as follows. Note that stream flow increases attributed to Lake McConaughy operational factors were lower as compared to Scenario 1 because Lake McConaughy was operated at a higher level.

Environmental Account: 15,000 AF/yr (same estimate used for Scenario 1)

Additional stream flow from reduced demand for irrigation/recharge diversions: 52,300 AF/yr

Lake McConaughy evaporation savings: 6,400 AF/yr

Additional stream flow from reduced McConaughy storage: 5,100 AF/yr

Average Annual Platte River Flow at Grand Island during Times of Shortage. During times of shortage, flows were simulated to increase at Grand Island by 118,000 AF/yr in Scenario 2. The increase is very similar in magnitude to Scenario 1. Increases were similar in the two scenarios because during times of shortage, diversions for recharge into CNPPID were curtailed partially or completely under both scenarios, and, compared to Baseline Conditions, a similar amount of water was made available for storage or stream flow enhancement. The Scenario 2 stream flow increase during times of shortage at Grand Island was 168,000 AF assuming that the Environmental Account would contribute an additional 50,000 AF/yr. Figure 4-8 shows flow improvements at Grand Island during times of shortage under Scenario 2. The results for Scenario 2 are very similar to the results for Scenario 1 in that annual flows at Grand Island showed significant improvement during shortage periods.

Hydropower Output. Hydropower output in Scenario 2 was simulated to increase by a total of 13.4 MKWH. Total hydropower generation under Baseline Conditions was simulated to be 455 MKWH on average. The additional hydropower output simulated under Scenario 2 therefore represents a 3% increase. As in Scenario 1, hydropower output decreased at Kingsley Dam because Lake McConaughy operated at a lower level. More water flowed down the river and passed through the North Platte Hydro and the CNPPID hydropower facilities, which offset the decreases from the Kingsley Dam unit. The increase in overall hydropower output in Scenario 2 could result in additional revenues of $350,000 to $540,000 per year on average given the assumptions on value of hydropower presented in Section 4.2.

Effects to monthly stream flows at Grand Island under Scenario 2 are shown in Figure 4-9. The figure shows that under Scenario 2, monthly average flows during shortage at Grand Island were generally higher than or similar to flows under Baseline Conditions.

4.3.4.2 Groundwater Effects

Groundwater effects from Scenario 2 were evaluated using monthly recharge amounts for the E-65 Canal system and crop consumptive demands for both the E-65 and Phelps Canal systems. Figure 4-10 shows the results of the COHYST model assessment of Scenario 2.

At the end of the 50-year modeling period, the COHYST model simulated up to 20 feet of mounding potential in the immediate vicinity of the E-65 canal system as well as up to 40 feet of potential water table decline within the groundwater mound in the vicinity of the eastern Phelps Canal system. Again, it is not likely that either the 20-foot increase or the 40-foot decrease would occur in a conjunctive water management scenario because of other optimization and mitigating factors described under the Scenario 1 groundwater modeling results. However, the integrated conjunctive use modeling does suggest that some level of recharge will need to occur in the Phelps Canal service area to maintain the groundwater mound; although additional groundwater pumping and beneficial use of the groundwater mound could help preserve valuable cropland and could prevent property damage from seepage and


4-12

high water tables. It is likely that recharge will need to be distributed across the Phelps Canal service area in some amount for an optimal solution. Although such a scenario was not discretely simulated for this scope of work, it is likely that such a conjunctive water management solution will provide for more water both to the Platte River system flows as well as regional aquifer reserves.


Scenario 3 used the results of Scenario 1 to evaluate the potential benefits of an additional storage facility upstream of Grand Island. The analysis was conducted at a very conceptual level using a spreadsheet tool to simulate the operations of the potential storage facility. The spreadsheet tool assumed that water would be stored in the facility during months when the results of Scenario 1 showed stream flow excesses at Grand Island. Water was then released from the facility when flows at Grand Island were below FWS targets. The spreadsheet tool accounted for maximum storage capacity limits and storage losses (based on Elwood Reservoir monthly storage losses from OPSTUDY) and was run on a monthly basis. Limitations on inflow or outflow rates were not included in the analysis. Facilities with capacities of 40,000 AF and 250,000 AF were assessed using the spreadsheet tool.

4.3.5.1 Surface Water Effects

The results of the conceptual analysis for a 40,000 AF and a 250,000 AF storage facility are shown in Figure 4-11. In the early years of Scenario 3, excess stream flows at Grand Island were seldom present, and water for storage was unavailable. Several years show approximately equal benefit for both the large and small storage facility because excess flows were present, but not in large volumes. In some years, excess flows were abundant and the larger storage facility provided more benefit than the smaller facility because it was able to capture and retime more of the excess flow. In later years of the simulation, more excess flows were available for retiming, and both storage facilities provided a means to increase flows at Grand Island with the larger facility providing more benefit than the smaller facility.

Figure 4-12 shows the Baseline Conditions and Scenario 1 results in Figure 4-4 (shown with semitransparent lines) but also includes flows at Grand Island during times of shortage with the 250,000 AF storage facility (Scenario 3 results). The figure shows many years when the additional storage facility is able to capture excesses and then release the stored water to boost stream flow during times of shortage.

It should be noted that the iterative nature of the modeling process made it difficult to optimize flows at Grand Island, and sometimes the simulation of conjunctive water management under Scenario 1 produced more flow than necessary to meet target flows during times of shortage at Grand Island. In these cases, the storage facility reduced flows at Grand Island and acted as a fine tuning mechanism.

On an average annual basis, the 40,000 AF storage facility provided an additional 14,000 AF of stream flow improvement during times of shortage at Grand Island. The 250,000 AF facility provided an average of approximately 50,000 AF/yr of additional flow during times of shortage.

5-1

Section 5

Conclusions and Recommendations 5.1 Conclusions This study assessed the potential to use existing infrastructure for recharge, evaluated infrastructure needed to conduct a conjunctive water management program (i.e. new groundwater wells, additional recharge facilities, etc.) and estimated the effects and benefits of various conjunctive water management scenarios. The study shows that it is feasible from a technical perspective to alter aspects of CNPPID’s operations to retime, and thus increase the overall usable water supply in the basin. Table 5-1 summarizes the potential benefits of the conjunctive water management scenarios that were described in Section 4.

Table 5-1. Summary of Potential Conjunctive Water Management Program Benefits

Parameter Benefits of Conjunctive Water Management

Average annual Platte River flow at Grand Island


Average annual Platte River flow at Grand Island during times of shortage


(Potentially over 215,000 acre-feet/year with a 250,000 acre-foot storage facility)

Total Hydropower Generation Increase of 0.1 to 13.4 MKWH

(Increased revenues of potentially over $500,000 per year on average)

Several additional conclusions can be drawn from the results of the evaluations, and they are listed below. CNPPID irrigators can reliably use the groundwater mound for irrigation supply. By using the

groundwater mound as the primary source of water supply, CNPPID irrigators would be able to irrigate when needed and not be subject to potential surface water supply issues. The groundwater mound could be used and managed as a reservoir for irrigation water supply.

Existing infrastructure can be used for conjunctive water management. The existing canal and lateral system would be useful and would be the primary recharge facility in a conjunctive water management program. Other infrastructure, such as recharge basins, may also be beneficial for recharging in targeted locations or for diverting and delivering larger volumes of excess stream flow for recharge purposes.

Increased hydropower revenues could be realized. OPSTUDY modeling indicates that overall hydropower production in the central Platte River valley could increase under a conjunctive water management program.

Section 5 Conclusions and Recommendations

5-2

Lake McConaughy water levels could be managed for greater stability. By relying on the groundwater mound as another reservoir of irrigation water supply, CNPPID could gain additional flexibility in managing storage in Lake McConaughy, which could lead to greater stability in water levels. Modeling conducted for this study suggested that water levels could be maintained at relatively stable levels much of the time, but there are times (i.e. during flood conditions) when lake levels will climb to capacity.

Water can be provided to help return the Platte River below Lake McConaughy to fully appropriated conditions. NDNR is currently developing methodologies for determining the amount of water supply that would be needed to return over-appropriated basins back to fully appropriated. The stream flow benefits derived from a conjunctive water management program would be very beneficial for returning to fully appropriated status in the Platte River basin.

The PRRIP could benefit significantly from this program. Surface water modeling conducted for this study suggested that significant stream flow increases could be realized during times of shortage. Additional stream flows derived from this conjunctive water management program resulting from the retiming of diversions, releases from storage, etc., would potentially help Nebraska and the basin states meet the first increment requirements of the PRRIP.

5.2 Recommendations The conclusions of this conceptual-level study indicate that the conjunctive water management program described in this report could greatly benefit CNPPID, stakeholders in the central Platte River valley, and the State of Nebraska. The proposed conjunctive water management program warrants more research and collaboration among parties that might participate in or benefit from the program. Recommendations for future activities to advance this program are listed below:

Refine the modeling analysis. This study was conducted at a conceptual level and the analysis tools had several limitations. Limitations and recommendations for overcoming the limitations are described below:

The surface water analysis tools were limited in their ability to optimize stream flow, storage in Lake McConaughy and in the groundwater mound, and hydropower production. Optimization modeling could be conducted to more precisely estimate the benefits of conjunctive water management. It is possible that optimization modeling would reveal additional stream flow, storage and hydropower benefits.

Groundwater changes and effects were estimated on a relative and not an absolute basis. The objective of the groundwater modeling was to identify locations in CNPPID’s service area where it would be difficult to maintain the groundwater aquifer in a conjunctive water management program. The objective was not to identify specific levels of drawdown or water table rise. Additional groundwater modeling needs to be conducted to identify actual water levels changes that may occur under a conjunctive water management program.

Effects to Platte River gains were roughly approximated and did not reflect the effects of periodic rising and lowering of the groundwater mound. Integrated surface and groundwater modeling tools would be useful in dynamically modeling the effects of groundwater mound management on the Platte River.

The modeling analysis did not include the drought of the early 2000s. The conjunctive water management program should be evaluated against the dry conditions experienced during this drought.

Conclusions and Recommendations Section 5

5-3

Develop operating rules. The modeling analyses conducted for this study were based on several assumptions regarding how the conjunctive water management program would be operated. CNPPID, NPPD, the PRRIP, and other stakeholders who could benefit or be impacted by the program should be engaged and should collaborate to develop and refine program objectives and operating rules. It is likely that additional scenarios of conjunctive water management will be developed through this process.

Assess current and future infrastructure needs. This study identified potential benefits from additional storage and recharge infrastructure and facilities, but the facilities are undefined in terms of actual locations, size, timing of need, etc. Additional assessments of needed infrastructure should be conducted once more refined modeling is performed and operating rules and goals are developed. In addition, field investigations could be conducted on existing canals and laterals to refine understandings of the spatial differences and magnitudes in recharge potential throughout the system. More refined modeling analyses of existing infrastructure and the aquifer may identify areas where additional infrastructure would be needed.

Explore legal, environmental, and socioeconomic considerations. The analysis in this report focused on technical aspects of a conjunctive water management program. Legal, environmental, socioeconomic and other considerations should be evaluated as the conjunctive water management program develops.

6-1

Section 6

Limitations This document was prepared solely for the Client in accordance with professional standards at the time the services were performed and in accordance with the contract between the Client and Brown and Caldwell dated June 30, 2010. This document is governed by the specific scope of work authorized by the Client; it is not intended to be relied upon by any other party except for regulatory authorities contemplated by the scope of work. We have relied on information or instructions provided by the Client and other parties and, unless otherwise expressly indicated, have made no independent investigation as to the validity, completeness, or accuracy of such information.

Further, Brown and Caldwell makes no warranties, express or implied, with respect to this document, except for those, if any, contained in the agreement pursuant to which the document was prepared. All data, drawings, documents, or information contained this report have been prepared exclusively for the person or entity to whom it was addressed and may not be relied upon by any other person or entity without the prior written consent of Brown and Caldwell unless otherwise provided by the Agreement pursuant to which these services were provided.

REF-1

References Blomquist, W.A., E. Schlager, T. Heikkila, Common Waters, Diverging Streams. Linking Institutions to Water Management

in Arizona, California, and Colorado. Resources for the Future, Washington DC, 2004.

Central Nebraska Public Power and Irrigation District. Irrigation Division. http://www.cnppid.com/Irrigation_Division.htm, December 2, 2011.

Howell, T.A. Encyclopedia of Water Science. “Irrigation Efficiency”. Marcel Dekker, Inc. New York, NY. 2003. pp 467-472

Nebraska Department of Natural Resources, Platte River Conjunctive Management Study, http://dnr.ne.gov/PlatteRiver/platteriverstudy.html

Nebraska Public Power District. Small Scale Renewable Resources. http://www.nppd.com/about-us/power-plants-facilities/renewable-energy/small-scale-renewable-resources/. 2012.

Nebraska Public Power District. Request for Proposal no. 10005, Power Purchase from Small Renewable Electric Generation Projects. April 1, 2010.

Peterson, Steven M. Groundwater Flow Model of the Eastern Model Unit of the Nebraska Cooperative Hydrology Study (COHYST) Area, Approved by Technical Committee, November 13, 2007.

Platte River Recovery Implementation Program. Final Platte River Recovery Implementation Program, Water Plan, Appendix B, FWS’ Use of the Central Platte Opstudy Model in Computing Reductions in Shortages to Target Flows. 2006.

60%240,000

Figure 3‐2. Annual Delivery Losses and Delivery Efficiency for CNPPID Irrigation System

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Figure 3‐3. Summary of Average 1954‐2002 Historical Monthly Delivery Loss Amounts from CNPPID Irrigation Distribution System

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Figure 3‐5. Summary of CNPPID Water Budget

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Figure 4‐2. Effects of Conjunctive Water Managment Scenario 1 on Annual Flows at Grand Island

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Figure 4‐4. Effects of Conjunctive Water Management Scenario 1 on Average Monthly Flows at Grand Island During Times of Shortage

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Figure 4‐6. Use of Groundwater Mound in Combination with Lake McConaughy Storage under Conjunctive Water Managment Scenario 2

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Figure 4‐7. Effects of Conjunctive Water Managment Scenario 2 on Annual Flows at Grand Island

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Figure 4‐9. Effects of Conjunctive Water Management Scenario 2 on Average Monthly Flows at Grand Island During Times of Shortage

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Figure 4‐11. Additional Increase in Flow at Grand Island (Beyond Scenario 1 Improvements) During Times of Shortage Due to Additional Storage of Various Capacities

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Conjunctive Water Management Conceptual Study

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