1697 Cole Boulevard, Suite 200

Golden, Colorado 80401

Report Addendum #1 Conjunctive Water Management Conceptual Study

Released September 24, 2013

i

Table of Contents List of Figures ............................................................................................................................................................. i 

List of Tables ............................................................................................................................................................... i 

List of Abbreviations .................................................................................................................................................. ii 1. Introduction .................................................................................................................................................... 1-1 2. Additional Surface Water Modeling .............................................................................................................. 2-1 

2.1  Changes to Modeling Inputs and Objectives ..................................................................................... 2-1 2.1.1  Minimum storage in Lake McConaughy .............................................................................. 2-1 2.1.2  Fluctuations in Lake McConaughy storage ......................................................................... 2-1 2.1.3  Recharge rates ...................................................................................................................... 2-1 

2.2  Modeling Results ................................................................................................................................. 2-2 3. Groundwater Modeling .................................................................................................................................. 3-1 

3.1  Superposition Modeling Approach and Initial Conditions ................................................................. 3-1 3.2  Recommendations for Additional Groundwater Modeling................................................................ 3-2 

4. Downstream Flow Improvements ................................................................................................................. 4-1 5. Limitations ...................................................................................................................................................... 5-1 6. References ................................................................................................................................................. REF-1 

List of Figures Figure 2-1. Use of Groundwater Mound in Combination with Lake McConaughy Storage under Conjunctive

Water Management - Revised Scenario 1

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

Figure 2-3. Effects of Conjunctive Water Management Revised Scenario 1 on Flows at Grand Island During Times of Shortage

Figure 2-4. Effects of Conjunctive Water Management Revised Scenario 1 on Average Monthly Flows at Grand Island During Times of Shortage

Figure 2-5. End-of-Month Storage in Lake McConaughy - Revised Scenario 1

Figure 4-1. Range of Average Monthly Improvement in Platte River Flow at Louisville during Times of Shortage under Revised Scenario 1

List of Tables Table 2-1. Summary of Revised Scenario 1 Stream Flow and Hydropower Effects ......................................... 2-3 

Table 4-1. Percentages of Additional Flow at Grand Island Expected to Reach Louisville .............................. 4-2 

Report Addendum #1

Conjunctive Water Management Conceptual Study Table of Contents

ii

List of Abbreviations

AF acre-feet

cfs cubic feet per second

CNPPID Central Nebraska Public Power and Irrigation District

COHYST Cooperative Hydrology Study

FWS U.S. Fish and Wildlife Service

MKWH million kilowatt-hours

PRRIP Platte River Recovery Implementation Program

yr year

1-1

Section 1

Introduction Brown and Caldwell released a report on August 15, 2012 describing a pre-feasibility study of enhancing the conjunctive management of surface and groundwater in the operations of the Central Nebraska Public Power and Irrigation District’s (CNPPID’s) irrigation system. The study examined the potential for changing the operations of CNPPID’s irrigation system to enhance the conjunctive use and overall utilization of surface and groundwater supplies. The study assessed potential benefits to Lake McConaughy and flow increases in the Platte River during times of shortage for the benefit of endangered species in the central Platte River region.

One of the report’s recommendations stated that several assumptions were used in the assessment of conjunctive water management, and stakeholders should be engaged to refine the program objectives and operational rules. While no formal engagement with stakeholders has taken place yet, several concerns and questions regarding the study’s assumptions and modeling have been raised by CNPPID staff since the release of the original report. Several of the concerns and questions can be generalized as follows:

The conjunctive water management project, as described in the report, may periodically result in lower-than-desirable storage levels in Lake McConaughy.

Lake McConaughy water levels may fluctuate more under the conjunctive water management proposal than they have historically.

The existing system of canals and laterals in CNPPID’s irrigation system cannot provide as much recharge as assumed in the analysis of conjunctive water management.

More explanation is needed to clarify how the groundwater model was used to assess potential effects on the existing groundwater mound.

The modeling does not include the drought period in the early to mid 2000s.

Brown and Caldwell agrees that the model should be extended to include this drought period. As described in the original report, the current version of the OPSTUDY model includes input data sufficient to run the model to 2002. Input data sets reflecting post-2002 conditions would need to be generated to extend the model to present day. Extending the input data sets was beyond the scope of the original pre-feasibility study, but it should be included in future studies of conjunctive water management. The modeling described in this addendum uses the same modeling period as the original report.

Brown and Caldwell conducted an additional modeling analysis using the OPSTUDY model to address concerns identified by CNPPID staff. In addition, Brown and Caldwell and CNPPID staff held several phone conversations to discuss questions related to groundwater modeling. This report addendum was written to describe the additional modeling analysis and results and to provide additional information regarding the groundwater modeling in the original pre-feasibility study. In addition, the addendum describes modeling results showing potential Platte River flow improvements farther downstream in the Louisville, Nebraska vicinity.

2-1

Section 2

Additional Surface Water Modeling In response to the concerns described in Section 1, Brown and Caldwell conducted additional analysis of potential conjunctive water management operations using the OPSTUDY model. Modeling input parameters and objectives were changed in accordance with the concerns and feedback provided by CNPPID. This section describes the inputs and objectives that were changed and how the changes affected modeling results.

2.1 Changes to Modeling Inputs and Objectives 2.1.1 Minimum storage in Lake McConaughy

The original pre-feasibility study included a modeling objective to operate Lake McConaughy generally between 800,000 acre-feet (AF) and 1,200,000 AF of storage. The success of meeting this objective was evaluated on an annual basis. The modeling in the original analysis generally met this objective on an annual basis. However, there were several individual months when storage in Lake McConaughy was less than 800,000 AF.

In the revised analysis described in this addendum, a more detailed assessment of Lake McConaughy storage volumes was conducted with the objective of preventing storage levels in Lake McConaughy from dropping below 800,000 AF in individual months.

2.1.2 Fluctuations in Lake McConaughy storage

A modeling objective of the pre-feasibility study was to evaluate the potential to operate Lake McConaughy at lower levels, which would provide an opportunity to capture and manage additional surface water during wet periods. A related objective was to minimize Lake McConaughy spills. If the lake is operated at a lower level than it has been in the past, there will be times when the lake level rises as a result of high inflows and storage of those inflows. The rise in the lake level would likely be more than it would have been in the past, but the amount of potential spillage would be lower than in the past.

The modeling objective of operating the lake at a lower level and storing additional inflow during wetter hydrologic periods was not changed. However, more attention was paid to operating the lake at a stable level during normal and dry periods. During wet periods, the amount of inflow to Lake McConaughy sometimes greatly exceeds the available storage, and storage amounts and lake levels will reach their maximum regardless of the objective to operate the lake at a lower level.

2.1.3 Recharge rates

The original pre-feasibility report provided estimates of monthly recharge rates based on an annual quantification of delivery losses and an average monthly distribution of diversions for irrigation. The original estimates assumed that monthly loss rates were proportional to diversions for irrigation, but the report acknowledged that seasonal changes in loss rates occur and that the estimates presented in the report were meant to provide a general understanding of delivery losses.

Figure 3-3 of the original report showed average monthly delivery losses in the range of 40,000 AF during July and August. In addition, the methodology described above suggested that monthly delivery losses could be as high as 60,000 AF in some specific months.

Report Addendum #1

Conjunctive Water Management Conceptual Study Section 2

2-2

Monthly recharge amounts used in the original modeling of conjunctive water management used the above monthly averages and potential maximums as general guidelines, but departed from those guidelines during wet periods when flows available for recharge were abundant. During several months when water available for recharge was abundant, amounts of water in excess of historical recharge amounts were designated for recharge in the modeling input files. The departures were allowed for reasons described below:

A portion of the high monthly amounts of recharge assumed in the modeling could, in practice, potentially be stored in CNPPID reservoirs or Lake McConaughy and delivered in subsequent months. In other words, not all of the water designated for recharge in a particular month in the modeling would necessarily need to be delivered to recharge in that month. More detailed modeling would need to be conducted to simulate the temporary storage and release of water designated for recharge.

Recharge basins could be constructed at strategic locations in CNPPID’s irrigation system to accommodate higher amounts of recharge.

Since the release of the original pre-feasibility report, CNPPID staff have stated that the maximum amount of monthly recharge that the existing canal/lateral system could achieve is 30,000 AF. In response, the modeling input files were changed to reflect a maximum monthly recharge potential of 30,000 AF. The management strategies described in the bullet list above were not used for the revised modeling analysis, but should be left open for consideration in future assessments of conjunctive water management.

2.2 Modeling Results The OPSTUDY model was used to re-evaluate Scenario 1 (see original report for a description of Scenario 1) using the changes to modeling objectives and inputs described above. The same modeling procedure used in the original analysis was used in the revised modeling. The procedure is described in Section 4.1.1.2 in the original report. The revised simulations were conducted by focusing on 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.

Several output parameters from OPSTUDY were examined to assess the results of the revised modeling with respect to surface water and hydropower effects. The primary output parameters from OPSTUDY used to evaluate the modeling 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 the revised modeling were compared with the corresponding OPSTUDY output for Baseline Conditions to assess simulated changes in those parameters under conjunctive water management.

Figure 2-1 illustrates how Lake McConaughy and the groundwater mound were used during the revised simulation. The bottom half of Figure 2-1 shows a bar chart representing annual average storage in Lake McConaughy in the Baseline Conditions model (blue bars) and as operated under Revised 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 that replenish the groundwater mound. Figure 2-1 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 2-1 shows recharge of the groundwater mound occurring during wet periods (i.e. the early 1970s, early 1980s, mid 1990s 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).

Report Addendum #1

Conjunctive Water Management Conceptual Study Section 2

2-3

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

The results of Revised Scenario 1 simulations with respect to stream flows and hydropower are described in Table 2-1. Further description and explanation of the parameters and output is provided in the discussion following Table 2-1.

Table 2-1. Summary of Revised Scenario 1 Stream Flow and Hydropower Effects

Parameter Change under Conjunctive Water Management Program

Stream Flow

Average Annual Platte River Flow at Grand Island Increase of 8,000 AF/yr

(23,000 AF/yr with Environmental Account)

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

Increase of 93,000 AF/yr

(143,000 AF/yr with Environmental Account)

Hydropower Output

Kingsley -6.5 MKWH/yr (decrease)

North Platte Hydro +1.5 MKWH/yr (increase)

Central Supply Canal (Jeffrey, J1, J2) +5.0 MKWH/yr (increase)

Total 0 MKWH/yr (no increase or decrease)

Average Annual Platte River Flow at Grand Island. Flows at Grand Island increased by 8,000 AF/year (yr)

on average under Revised Scenario 1. This is less than under the original Scenario 1 modeling, which resulted in 31,000 AF/yr in flow improvements at Grand Island. In Revised Scenario 1, slightly more water on average was diverted to recharge, which lowered the average annual amount of stream flow increase. In addition, savings in evaporative losses in Lake McConaughy and additional stream flow from reduced McConaughy storage were both less in Revised Scenario 1 because the lake was operated at a higher level.

Average Annual Platte River Flow at Grand Island during Times of Shortage. Under Revised Scenario 1, flows during times of shortage increased at Grand Island by 93,000 AF/yr on average. This is less than under the original Scenario 1 modeling, which resulted in 115,000 AF/yr flow improvement during times of shortage at Grand Island. The Environmental Account has been estimated to boost stream flows during times of shortage by 50,000 AF/yr on average. The combined effects of Revised Scenario 1 and the Environmental Account could increase stream flow during times of shortage at Grand Island by an average total of 143,000 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 2-3 illustrates how flows at Grand Island improved under Revised Scenario 1. The figure shows annual flows at Grand Island during shortage under Baseline Conditions and flows at Grand Island under Revised 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.

Report Addendum #1

Conjunctive Water Management Conceptual Study Section 2

2-4

Shortage condition flows under Revised Scenario 1 were estimated by summing the Revised Scenario 1 flows at Grand Island during those same months. Figure 2-3 shows flow improvements at Grand Island during times of shortage, which provides a key benefit to the Platte River Recovery Implementation Program (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 Revised Scenario 1 and the Environmental Account during times of shortage nearly exceeds the PRRIP’s first increment goal of stream flow shortage reductions.

The improvements to stream flow at Grand Island under Revised Scenario 1 were less than under the original Scenario 1 because the reduced amount of monthly recharge potential provided less opportunity to retime excess stream flow. In addition, operating Lake McConaughy at a higher level reduced the ability to release stored water to enhance stream flow during times of shortage. However, improvements to these results could potentially be achieved by incorporating some of the water management options described in Section 2.1.3. In addition, the modeling conducted to date has not been optimized and has been done at a pre-feasibility level. Additional, more detailed modeling focused on optimization may improve the results obtained to date.

Hydropower Output. Under Revised Scenario 1, Lake McConaughy was simulated to operate at a lower level than Baseline Conditions, which resulted in less hydropower output at Kingsley Dam. However, more water was passed down the river and through the North Platte Hydro and CNPPID hydropower facilities. Under Revised 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 was maintained under Revised Scenario 1 and neither increased nor decreased as compared to Baseline Conditions.

Effects to monthly average stream flow at Grand Island under Revised Scenario 1 were also examined. Figure 2-4 shows that average monthly flows at Grand Island during times of shortage increased in Revised Scenario 1 compared to Baseline Conditions.

Lake McConaughy monthly storage amounts in Revised Scenario 1 were held above 800,000 AF in all months except during 2002. During 6 months in 2002, the lake level dropped below the 800,000 AF threshold. During those months, however, the lake level was held higher than under Baseline Conditions. Under Baseline Conditions, Lake McConaughy dropped below 800,000 AF a total of 16 months. The Baseline Conditions model showed Lake McConaughy dropping below 800,000 AF for 6 months in 2002 and for 10 months at various times during the 1955 to 1957 timeframe. Figure 2-5 shows a comparison of end-of-month storage in Lake McConaughy under Revised Scenario 1 and Baseline Conditions. The figure also depicts the changes in end-of-month storage between the original Scenario 1 simulation and the Revised Scenario 1 simulation.

The annual change in storage amounts in Lake McConaughy was evaluated to understand how the variability of lake levels would potentially be affected by conjunctive water management under Revised Scenario 1. The water level in Lake McConaughy was intentionally operated at a lower elevation in Revised Scenario 1 than under Baseline Conditions so that additional storage space would be available in the lake to capture and manage high flows during wet hydrologic cycles. As a result, the lake level will naturally climb when the additional storage space is filled. While this provides a benefit for water management, it will likely result in more lake level fluctuation at the onset of wet conditions (when the lake fills) and when the hydrologic cycle returns to normal or dry conditions (when the lake is lowered to less than 1,200,000 AF of storage). The evaluation of annual changes in lake levels under Revised Scenario 1 and Baseline Conditions assessed the change in end-of-month storage from September to September for each year. In other words, the September end-of-month storage for each year was compared to the September end-of-month storage for the prior year.

Report Addendum #1

Conjunctive Water Management Conceptual Study Section 2

2-5

Two evaluations of annual changes in storage were conducted. One of the evaluations included all of the years in the simulation, and one evaluation focused on normal and drier hydrologic cycles. For the years 1952 through 2002 (all of the years in the simulation), the September to September average change in Lake McConaughy storage was 184,000 AF under Baseline Conditions and 197,000 AF under Revised Scenario 1. The higher fluctuation in Lake McConaughy levels when all years are included in the evaluation is reflective of the increase in storage amounts in response to wet hydrologic cycles described above. While the September to September change in Lake McConaughy storage amounts was greater under Revised Scenario 1 compared to Baseline Conditions, it should be noted that higher levels of fluctuation occurred because Lake McConaughy was able to store and manage additional inflow, which reduced spills and allowed that water to be released to recharge the groundwater mound or boost stream flows during times of shortage.

In the evaluation of normal and drier hydrologic cycles, the wet periods during 1952-1954, 1971-1975, 1983-1989, and 1997-2001 were not considered. On average, the Baseline Conditions model showed an average annual change in September to September end-of-month storage of 180,000 AF, and the Revised Conditions model showed a lower average annual change at 133,000 AF.

Another way to view Lake McConaughy storage fluctuations is to compare high and low storage levels that occur within a calendar year. Under Baseline Conditions, the average annual difference in storage amounts occurring within calendar years during the period of simulation was approximately 363,000 AF when all years of the simulation were included. Under Revised Scenario 1, the average annual difference in storage amounts was approximately 395,000 AF. The maximum difference in storage amounts in a calendar year under Baseline Conditions and Revised Scenario 1 were 740,000 AF and 859,000 AF, respectively. Again, Revised Scenario 1 storage level fluctuations were greater than Baseline Conditions when all years were considered because Lake McConaughy was operated at a lower level and rose and fell response to wet periods. When the wet periods described above were removed from the comparison, the average annual difference in storage levels for Baseline Conditions was 378,000 AF. The average annual difference in storage levels under Revised Scenario 1 was 347,000 AF when wet periods were eliminated, which was less than Baseline Conditions. The maximum differences in storage levels in a calendar year under Baseline Conditions and Revised Scenario 1 were 740,000 AF and 785,000 AF, respectively.

The OPSTUDY modeling code was not altered for the Revised Scenario 1 simulations. While the Revised Scenario 1 simulations focused on improving the proposed conjunctive water management program, additional modeling that seeks to optimize conjunctive water management performance, improves upon the assumptions, and incorporates revisions to operating rules based on input from stakeholders is recommended.

3-1

Section 3

Groundwater Modeling Companion groundwater simulations were not performed in conjunction with the additional OPSTUDY modeling discussed in the preceding section. However, the following section provides additional information on:

The use and application of the COHYST groundwater modeling tool for this project Details and reasoning for the proposed future groundwater modeling tasks and model refinements

The groundwater modeling performed during the original pre-feasibility study of conjunctive water management was based upon general groundwater superposition theory and provided estimates of relative effects of various water management practices. The questions posed by CNPPID, and presented above, will assist in further refining future groundwater model simulations and identifying optimal and readily-implementable conjunctive management strategies. Additional groundwater model and simulation refinements will be required to answer more detailed technical questions regarding the absolute magnitude of groundwater and surface water benefits and effects due to various water management strategies. However, given the limits of superposition modeling, the previously-performed groundwater simulations of conjunctive water management demonstrated that groundwater recharge in excess of natural conditions will maintain a stable groundwater mound underneath CNPPID irrigated lands.

3.1 Superposition Modeling Approach and Initial Conditions As previously documented in Brown and Caldwell’s pre-feasibility study report (see Sections 4.1.2.1 and 4.3.3.2) the COHYST groundwater modeling tool was used to assess relative changes to the groundwater table under two different conjunctive water management scenarios. In the original groundwater model runs, the only pumping and recharge applied to the model above natural, predevelopment conditions were based explicitly upon the net irrigation crop requirements and associated groundwater recharge via leakage from canals. Groundwater interactions with surface water flows in the Platte River were simulated via the MODFLOW River Package, identical to the approach taken by the source COHYST model. This modeling approach is called a superposition approach to groundwater modeling and is described in the USGS publication, Guidelines for Evaluating Ground-Water Flow Models (Reilly and Harbaugh, 2004). The USGS notes that superposition models are used to “evaluate only changes in stress and changes in responses.” It is an improvement upon using a basic and standard analytical model solution, but the results and conclusions drawn from such a modeling application should be limited to mathematically linear hydrologic systems. This modeling approach greatly reduces the amount of effort involved in developing a groundwater flow simulation or updated flow model, while still providing scientifically valid and “reasonably accurate” guidance in determining the relative effect of hydrologic changes (i.e. varying pumping and recharge rates) upon an aquifer system (Reilly and Harbaugh, 2004).

The use of the superposition modeling approach inherently means that effects upon groundwater reserves can only be assessed relative to a predetermined baseline. It is not appropriate to use this modeling approach to assess the absolute elevations of future groundwater conditions for a given water management strategy due to two significant limitations of superposition modeling: 1) additional transient (or varying over time) hydrologic sources or sinks (or “stresses”) adjacent to CNPPID irrigated lands were not considered, and 2) the initial conditions used for the groundwater simulations were based upon pre-irrigation or pre-development conditions. This approach has been used by the USGS in multiple applications, and is also an

Report Addendum #1

Conjunctive Water Management Conceptual Study Section 8

3-2

advancement upon the use of analytical solutions that a project such as this would have to employ in the past due to the lack of a tool such as the COHYST model. However, as detailed in the Brown and Caldwell 2012 report and discussed below, additional groundwater simulations are required in order to adequately assess the absolute change in groundwater levels (relative to mean sea level) and aquifer conditions under various, optimized conjunctive management scenarios. The modeling work performed previously should be used to assess whether or not a different water management condition will maintain elevated water levels, or reduce them below what would be the natural condition prior to cropland development. It should not be used to assess how much mounding will occur in the future relative to present day conditions, with the exception that it will stabilize above pre-developed conditions.

Superposition modeling is very effective at determining the relative benefit or negative effect to groundwater resources due to a change in hydrologic stresses. Although the results of this study cannot be compared to present day conditions without the updates described in the following section, they do conclusively indicate that an area of elevated groundwater level conditions will persist beneath CNPPID lands. Given the principles of this modeling approach, this stored groundwater will be present and available even considering the pumping adjacent to the CNPPID service area. Additional modeling, previously proposed at the conclusion of the original report, would further refine the optimal timing and distribution of groundwater recharge to further protect aquifer reserves while limiting the effects of excessive recharge and crop and property inundation and flooding.

3.2 Recommendations for Additional Groundwater Modeling Because this study was performed at a pre-feasibility level and not a design level, the COHYST groundwater model was not fully updated and calibrated to present day conditions and expanded with projected demands and climatic conditions for the next 50 to 100 years. Such an update would be required to estimate the degree of groundwater mounding likely to occur in the future under an optimal water management scenario. Although results from the superposition modeling noted that recharge above natural conditions will maintain elevated water levels in the aquifer system, the degree of this mounding and stabilized groundwater levels were not explicitly estimated. As noted in Section 5.2 of the original project report, the objective of the groundwater modeling performed the pre-feasibility phase of this study was primarily to identify locations in CNPPID’s service area where it would be difficult to adequately manage local aquifer conditions under a conjunctive management scenario. The application of the COHYST model was very effective in assessing this at the regional scale. In order to accurately estimate future levels of groundwater rise or drawdown under a variety of climatic and water management scenarios and at more local spatial resolutions, the current version of the Eastern Unit of the COHYST model should be refined, recalibrated through the drought of the early 2000s, and used to perform a series of simulations to identify the most optimal and beneficial conjunctive management program for both the local hydrology and the crop producers who rely upon it.

Additional input from CNPPID, local producers, project stakeholders, and other qualified and interested parties will continue to enhance this study and refine future groundwater modeling towards the most realistic and optimally designed solutions that will protect the interests of both groundwater and surface water users throughout the Platte River corridor. In addition, future infrastructure enhancements for groundwater recharge and water conservation were not considered during the groundwater modeling. Future research and consideration of new infrastructure will provide a greater degree of freedom in assessing the potential hydrologic and economic benefits of conjunctive water management relative to the potential costs of construction.

4-1

Section 4

Downstream Flow Improvements Revised Scenario 1 modeling output was examined to evaluate potential Platte River flow improvements in eastern Nebraska resulting from the conjunctive water management activities proposed Revised Scenario 1. Platte River flow improvements in eastern Nebraska would be beneficial for a number of reasons. For example, the cities of Lincoln and Omaha operate wellfields in the Platte River alluvial aquifer near Ashland, Nebraska. These wellfields depend on Platte River flows to recharge the alluvial aquifer. If insufficient Platte River flow is available for recharge, the performance and output of the wellfields can be diminished. An additional example concerns in-stream flow rights; the Nebraska Game and Parks Commission holds in-stream flow rights on the lower Platte River for the protection of endangered species (pallid sturgeon). Improvements to flows in the Platte River due to upstream conjunctive water management activities would potentially be beneficial to endangered species.

OPSTUDY estimates Platte River flows at a couple locations downstream of Grand Island. Estimated flows at Louisville, Nebraska are included in OPSTUDY output and were used as the basis for understanding potential benefits of conjunctive water management activities for lower Platte River water users. OPSTUDY estimates flows at Louisville by adding historical river gains/losses to the calculated flows at Grand Island, Nebraska. However, the methodology used by OPSTUDY does not account for the downstream conveyance loss (seepage and evaporation) of additional flows at Grand Island.

Conveyance losses between Grand Island and Louisville were estimated in studies associated with the PRRIP. HDR Engineering, Inc. recently conducted a study entitled “Lower Platte River Stage Change Study Final Protocol Implementation Report” (HDR, 2009) The objective of the HDR study was to investigate the potential effects of PRRIP water management activities on the lower Platte River. The HDR study used and extended a conveyance loss analysis originally developed by the FWS. The original FWS study used a daily flow tracking and accounting model to estimate evaporation and seepage losses in the Platte River. A key objective of the study was to estimate the proportion of potential flow enhancements from the PRRIP at Grand Island that would have been expected to reach Louisville under conditions that existed during the evaluation period. The HDR study extended the evaluation period used in the original FWS study to generally include the years 1975 to 2008. The reader is referred to the HDR and FWS studies for more information and details about their respective methodologies, inputs, results, etc.

The HDR study includes figures that show, on a monthly basis, ranges of the estimated percentage of various levels of PRRIP program water at Grand Island that would be expected to reach Louisville. The figures were developed for flow rates of PRRIP program water of 100 cfs, 500 cfs, and 1000 cfs. The different levels of program water were evaluated, because it is likely that a higher proportion of program water would be lost between Grand Island and Louisville if the flow rate of program water is low. Less program water would potentially be lost at higher flow rates. In addition, high and low ranges of potential percentages of water reaching Louisville were presented in the figures. The high and low ranges were based on the 25th and 75th percentiles of losses estimated in HDR conveyance loss study. Tabular information corresponding to the values depicted in the figures were not included in the HDR report. For the purposes of this addendum, values were estimated from the HDR report figures and are shown in Table 4-1 below.

Report Addendum #1

Conjunctive Water Management Conceptual Study Section 8

4-2

Table 4-1. Percentages of Additional Flow at Grand Island Expected to Reach Louisville

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Low Range 100 cfs 53 61 72 77 65 38 10 12 22 53 68 63

500 cfs 53 61 72 82 71 59 36 39 49 62 71 63

1000 cfs 52 61 72 84 75 68 48 51 60 67 73 62

High Range 100 cfs 90 88 92 91 88 84 64 63 78 83 89 90

500 cfs 91 88 92 92 89 85 68 70 82 84 90 91

1000 cfs 92 88 93 94 90 87 72 75 84 85 92 92

The percentages in Table 4-1 were used to adjust the OPSTUDY output with respect to flow improvements at Louisville. In months when flow improvements at Louisville resulted from additional flow at Grand Island from Revised Scenario 1 operations, the flow improvements were reduced based on the flow rate of the improvement and month. For flow rate improvements between 0 and 250 cubic feet per second (cfs), the percentages in Table 4-1 associated with 100 cfs of additional flow were used. The percentages associated with 500 cfs of additional flow were used to adjust flow improvements between 250 cfs and 750 cfs, and the percentages associated with 1000 cfs were used to adjust flow improvements over 750 cfs.

The resulting ranges of potential flow improvements at Louisville are shown in Figure 4-1. The figure shows potential average monthly Platte River flow improvements at Louisville during times of shortage. As shown in the figure, flow improvements between 250 cfs and 550 cfs could potentially occur at Louisville in July and August during times of shortage as a result of Revised Scenario 1 operations. Flow improvements during shortage in other times of the year could result as well. The amount of flow improvement at Louisville under Revised Scenario 1 tended to be highest in July and August, lowest in September through January, and relatively moderate in February through June. The average annual pattern of flow improvement at Louisville was a result of operations of Revised Scenario 1, which were aimed at providing water for irrigation, managing the groundwater mound and Lake McConaughy, and enhancing stream flow at Grand Island. Annual patterns of flow improvement needs at Louisville were not considered in the development of Revised Scenario 1, but could be a consideration in future modeling runs.

5-1

Section 5

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

Section 6

References HDR Engineering, Inc. Lower Platte River Stage Change Study, Final Protocol Implementation Report. Prepared for the Platte

River Recovery Implementation Program. Version 1.0. December 2009.

Reilly, T.E. and Harbaugh, A.W., Guidelines for evaluating ground-water flow models: U.S. Geologic Survey Scientific Investigations Report 2004-5038, 2004, p. 4.

‐2200

‐2000

‐1800

‐1600

‐1400

‐1200

‐1000

‐800

‐600

‐400

‐200

0

200

400

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

2,200

2,400

2,600

Amou

nt re

charged (+) o

r bo

rrow

ed (‐) from m

ound

 (KAF

)

Annu

al Average

 Lake McCon

aughy Storage (KAF

)

Year

Figure 2‐1.  Use of Groundwater Mound in Combination with Lake McConaughy Storage under Conjunctive Water Management ‐ Revised Scenario 1

Baseline Condition

Conjunctive Water Management Revised Scenario 1

Replacing groundwater

Borrowinggroundwater

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

4,500

Annu

al Flow at G

rand

 Island

 (KAF

)

Year

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

Baseline ConditionConjunctive Water Management Revised Scenario 1FWS Target flow

0

200

400

600

800

1,000

1,200

Annu

al Flow Volum

e During Shortage

 (KAF

)

Year

Figure 2‐3.  Effects of Conjunctive Water Management Revised Scenario 1 on Flows at Grand Island During Times of Shortage 

Flows at GI During Shortage ‐ Baseline Flow at GI During Shortage ‐ Revised Scenario 1

Avg Annual Flow at GI During Shortage ‐ Baseline Avg Annual Flow at GI During Shortage ‐ Revised Scenario 1

FWS Target Increase During Shortage

0

20

40

60

80

100

120

140

160

180

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Flow

s (KAF

)Figure 2‐4.  Effects of Conjunctive Water Management Revised Scenario 1 on Average 

Monthly Flows at Grand Island During Times of Shortage

Baseline Condition

Conjunctive Water Management Revised Scenario 1

FWS Target Flows

0

100

200

300

400

500

600

700

800

900

1,000

1,100

1,200

1,300

1,400

1,500

1,600

1,700

1,800

1,900

Mon

thly EOM Storage

 (KAF

)

Year

Figure 2‐5.  End‐of‐Month Storage in Lake McConaughy ‐ Revised Scenario 1

Baseline ConditionRevised Scenario 1Original Scenario 1

0

100

200

300

400

500

600

700

800

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Average Platte River Flow Im

provem

ent (cfs)

Figure 4‐1.  Range of Average Monthly Improvement in Platte River Flow at Louisville during Times of Shortage under Revised Scenario 1

Low

High