CE Database subject headings: Climate change; Colorado River; Water reclamation; Water resources; Water supply; River basins;Streamflow; Hydrologic models.
Author keywords: Climatic changes; Colorado River; Resource management; Reclamation; Water resources; Water supply;San Juan River Basin; Streamflow.
Introduction
Water resource managers have traditionally relied on past observa-tions of hydrology to project future operations; however, as theeffects of climate change are realized, past observations of temper-ature, precipitation, and streamflow may no longer be an adequaterepresentation of the future (Milly et al. 2008). Therefore, incorpo-ration of alternative hydrologic scenarios, specifically those thatincorporate the potential effects of climate change, would bebeneficial to water resource managers (Brekke and Prairie 2009).Miller et al. (2011) explored potential hydrologic variability underalternative climate scenarios using the National Weather Service(NWS) River Forecasting System (RFS) over Colorado Riverheadwater basins. Although Miller et al. (2011) addressed hydro-logic uncertainty related to changing climate, the study did notexplicitly address whether a different hydrologic model wouldhave significantly affected results. In this research, streamflow pro-jections are developed using both the variable infiltration capacity(VIC) model and the NWS RFS provided by the Colorado BasinRiver Forecasting Center (CBRFC) to force a modified version ofthe Bureau of Reclamation’s (Reclamation) long-term policymodel. These streamflow projections are developed by using 112identical projections of future climate (i.e., temperature and precipi-tation) derived from global climate models (GCMs). A comparison
1Hydrologic Engineer, United States Bureau of Reclamation, LowerColorado Region, P.O. Box 61470, ATTN: LC-4634, Boulder City, NV89006 (corresponding author). E-mail: [email protected]
2Civil Engineer, United States Bureau of Reclamation, Lower ColoradoRegion, 1777 Exposition Dr., Suite 113, ATTN: LC-4004, Boulder, CO80302; and Center for Advanced Decision Support for Water and Environ-mental Systems (CADSWES), Univ. of Colorado, Boulder, CO 80309.E-mail: [email protected]
3Associate Vice President for Interdisciplinary Research, Univ. ofNevada, Las Vegas, 4505 Maryland Parkway, Box 451087, Las Vegas,NV 89054. E-mail: [email protected]
4Hydraulic Engineer, United States Bureau of Reclamation, UpperColorado Region, 1777 Exposition Dr., Suite 113, ATTN: UC-246,Boulder, CO 80302. E-mail: [email protected]
5Hydrologic Engineer, United States Bureau of Reclamation, UpperColorado Region, 125 South State St., ATTN: UC-432, Salt Lake City,UT 84138. E-mail: [email protected]
6Student, Univ. of Arizona, Dept. of Hydrology and Water Resources,1133 E. James E. Rogers Way, Room 122, P.O. Box 210011, Tucson, AZ85721. E-mail: [email protected]
of projected operations within the San Juan River Basin derivedusing streamflow projections from two different hydrologic modelsis then quantified to evaluate whether hydrologic model choice hasa significant effect on operational projections. The goal and con-tribution of this study is to evaluate whether differences betweenhydrologic models used to develop streamflow projections fromprojected future climate information ultimately affects operationaldecision making for water resource managers.
San Juan River Basin
The San Juan River Basin within the Upper Colorado River Basin iscurrently adaptively managed. The San Juan River Basin spansover the Four Corners area of the United States, including portionsof Colorado, Utah, Arizona, and New Mexico within the UpperColorado River Basin. Reclamation operates the Navajo andVallecito dams and reservoirs within the basin to regulate flowfrom the San Juan River and its tributaries to the Colorado River(Fig. 1). The San Juan River Basin is operated as described inReclamation’s Final environmental impact statement, NavajoReservoir operations, Navajo unit–San Juan River, New Mexico,Colorado, and Utah (U.S. Dept. of the Interior 2006). These oper-ations are in agreement with flow recommendations made bythe San Juan River Basin Recovery Implementation Program(SJRBRIP). The goals of the SJRBRIP are to protect tribal waterinterests and water use and development in the area while alsomaintaining water releases to recover two endangered fishspecies, the Colorado pikeminnow and razorback sucker, and theirhabitat in accordance with the Endangered Species Act (ESA)(San Juan River Basin Recovery Implementation Program BiologyCommittee 1999).
Flow recommendations established by the SJRBRIP are fol-lowed while also protecting the purposes of the Colorado RiverStorage Project Act and Indian trust assets. The Navajo IndianIrrigation Project (NIIP) diverts water from the Navajo reservoirat an intake elevation of 1,825.75 m (5,990 ft) when storage isapproximately 817 106 m3 (0.662 106 acre-ft). During the winter,the reservoir can be lowered to 1,824.22 m (5,985 ft) with approx-imately 772 106 m3 (0.626 106 acre-ft) in storage as long as thereservoir recovers prior the beginning of the irrigation season(U.S. Dept. of the Interior 2006). Water resource development
planning and Reclamation’s ability to comply with Section 7 ofthe ESAwithin the San Juan River Basin and subsequent effects tothe SJRBRIP flow recommendations are evaluated using multipleReclamation planning models. There is a need to explicitly incor-porate climate change information in the development of water useand supply within the basin under varying hydrologic scenarios.
This study will first present a brief description of the climatedata set used to force the models described to develop projectionsof streamflow. Next, a brief description of both the VIC and NWSRFS hydrologic models utilized for deriving projections of futurestreamflow, followed by a brief description of Reclamation’sColorado River Simulation System (CRSS) long-term planningmodel. A description of the CRSS methodology follows, and thenthe results of forcing CRSS with the derived streamflow from eachof the hydrologic models are offered. The study concludes with adiscussion of what was learned from the results.
Data
Temporally Disaggregated BCSD Climate Projections
Reclamation has recently worked with Santa Clara University(SCU) and Lawrence Livermore National Labs (LLNL) to developand make available an archive of 112 bias-corrected and spatiallydisaggregated (BCSD) (Wood et al. 2004) projections of temper-ature and precipitation from the World Climate Research Program’s(WCRP’s) Coupled Model Intercomparison Project phase 3(CMIP3) multimodel data set (Meehl et al. 2007). To developstreamflow projections under changing climate conditions, boththe RFS and VIC hydrologic models were forced by all 112projections of temperature and precipitation from the BCSDclimate data from the WCRP’s CMIP3 multimodel data set (avail-able at: http://gdo-dcp.ucllnl.org/downscaled_cmip3_projections).
The archive hosts monthly projections of temperature and pre-cipitation data available at a 1=8° (approximately 12 km or 7.5 mi)grid cell resolution, which is more applicable and suitable forregional hydrologic analysis than raw GCM output at the2° (approximately 200 km or 124 mi) grid cell resolution. Thesmaller grid cell resolution captures more of the topographiceffects on climate that are important to regional analysis thanresolution at a global scale.
The VIC model operates at a daily timestep; as such, temporaldisaggregation of monthly WCRP CMIP3 data over the ColoradoRiver Basin is required. Temporal disaggregation of the monthlydata was accomplished by scaling historical daily precipitationor shifting historical daily temperature data to match projectedmonthly time series data (Wood et al. 2004). Daily precipitationand temperature time series have been derived for the entire spatialand temporal extent of the monthly BCSD data set. The NWS RFSmodel operates at a 6-h time step, requiring further disaggregation.Precipitation and temperature time series at the 6-h time step havebeen developed over the San Juan River Basin for the NWS RFSand are presented in Miller et al. (2011).
Hydrologic Models
Reclamation utilizes multiple models and input from numerousagencies and stakeholders to manage the Colorado River System.Reclamation is required to use forecasts of streamflow developedfrom the NWS RFS by the CBRFC in the development ofmid term (1 to 24 months) operational forecasts. The VIC modelhas been used in numerous studies evaluating water supplyunder changing conditions over the Colorado River Basin [e.g.,Christensen et al. (2004), Christensen and Lettenmaier (2007),
Fig. 1. San Juan River Basin; Reclamation operates reservoirs withinthe basin in compliance with the SJBRIP
Tang and Piechota (2009)]. Reclamation has incorporated the VICmodel into its Colorado River Basin Water Supply and DemandStudy (U.S. Dept. of the Interior 2009). Both models have beencalibrated to historical natural streamflow; that is, streamflow thathas been adjusted to remove anthropogenic influences such asreservoir regulation and consumptive use. The decision to usethese two hydrologic models is developed a priori; that is, the useof these models is dictated through previous agreements withColorado River Basin stakeholders (as is the case with theNWS RFS) or in current use in other research efforts (as is the casewith the VIC model).
VIC Hydrologic Model
The VIC model (Xu et al. 1994) is a large-scale, grid-basedhydrologic model. VIC has been used in previous studies to helpassess climate effects on the Colorado River Basin (Christensenet al. 2004; Christensen and Lettenmaier 2007). Reclamation col-laborated with the environmental consulting firm AMEC to em-ploy the VIC model developed by Christianson and Lettenmier(2007) for the Colorado River Basin. To produce one set of thestreamflow projections utilized in this analysis, VIC was forcedby temporally disaggregated temperature and precipitation datadescribed previously in this study. A more detailed descriptionof the process can be found as part of the Colorado River BasinSupply and Demand Study (Basin Study) (U.S. Dept. of theInterior 2009).
VIC computes total evapotranspiration from three separatetypes of evaporation and transpiration processes that are depen-dent on land cover parameters, whereas snow accumulationand snow melt are explicitly modeled by coupling VIC with anenergy-balance-approach snow accumulation model. VIC canbe coupled with a GCM or run independently (as it was to producethe data used in this study) using temperature and precipitationdata to force the model. Direct and subsurface runoff and evapo-ration are simulated. By using the direct surface runoff and a flowrouting model, flow at a specific point (gauge) can be derived(Nijssen et al. 1997).
National Weather Service Hydrologic Model
The lumped, hydrologic forecasting system employed by the NWSincorporates numerous models; however, over the Colorado RiverBasin, there are two primary models on which hydrologic forecastsof streamflow are most dependent: the Sacramento Soil MoistureAccounting (SAC-SMA) model (Burnash et al. 1973) and the SnowAccumulation and Ablation Model (SNOW-17) (Anderson 1973,2006). The forecasting system has predominately been appliedto provide inflow forecasts for reservoirs managed by Reclamationin the Upper Colorado River Basin, of which the San Juan is asubbasin, over a 3 to 7 month horizon. This forecasting system,provided and calibrated by the CBRFC, was used to developone set of the streamflow projections used in this study.
Effects to evapotranspiration under changing climate have rarelybeen considered when using hydrologic models and projections offuture climate (Brekke and Prairie 2009). The CBRFC currentlyrelies on values of evapotranspiration demand unique to eachmonth when running the RFS; for example, all Januarys have iden-tical evapotranspiration demands throughout the length of themodel run. The NWS RFS provided by the CBRFC was modifiedto incorporate changes to evapotranspiration as a function ofmonthly average projected temperature. A detailed explanationregarding the derivation of evapotranspirative demand is providedin Miller et al. (2011).
Lumped Versus Distributed Projections of HydrologicInflow
The RFS and VIC hydrologic models presented in this paper usethe same temporally disaggregated BCSD data set to develop pre-cipitation and temperature input. However, it is important to notethat although information from the temporally disaggregatedBCSD data set was able to be directly input into the VIC model,precipitation and temperature information was aggregated andadjusted as previously described Miller et al. (2011) for input intothe NWS RFS. The VIC model uses the Penman-Monteith equationto formulate potential evapotranspiration over the study area; toincrease consistency between the NWS RFS and VIC models,an explicit time series of evapotranspiration demand as a functionof temperature was derived (Miller et al. 2011). While both hydro-logic models report monthly results, temperature and precipitationinput into the VIC model is at the daily time step, whereas temper-ature and precipitation input into the RFS is at the 6-hourly time-step. Evapotranspiration input into the RFS is at the daily timestep.
In VIC, leaf area index, canopy resistance, and the relative frac-tion of roots in each soil layer are used to characterize the type ofland cover for each grid cell. The specification of either the maxi-mum soil moisture content of the top layer of soil or the maximuminfiltration capacity help determine what portion of a precipitationevent becomes surface runoff (Xu et al. 1994).
Land surface and hydrologic characteristics are described overeach elevation band within each catchment for the NWS RFS pri-marily through input parameters for the SAC-SMA and SNOW-17models and remain constant over the duration of the model run.
Hydrologic Model Validation and Secondary BiasCorrection
A verification run of both hydrologic models was performed to de-termine the ratios for a secondary bias correction that is applied atthe monthly level to remove any bias from the hydrologic models.The verification runs force the hydrologic models with observedhistorical climate data; the modeled historic stream flow is thencompared with historic natural flow (Prairie and Callejo 2005)to determine the secondary bias-correction ratios. VIC was forcedwith historical precipitation and temperature for 1950–1999(Maurer et al. 2002), while the RFS model was forced with histori-cal data from 1976–1999. The use of different historical data sets isdue to the difference in the model structures (lumped versusdistributed) and the available data that can be readily used bythe models.
In both cases, monthly average flows are computed for the veri-fication run and the historic period. Ratios are then computed toscale the verification run to match the historic monthly averages.After the secondary bias correction, the monthly statistics of theverification run match the historical monthly values, though thereare still differences in the total annual volume. Fig. 2 shows boththe annual volume and the monthly averages for the total naturalflow at Bluff, UT. For the monthly averages, the bias-correctedrun collapses onto the historical averages. Similar results wereobserved at Archuletta, NM. The ratios are used to bias correctthe stream flow projections produced by VIC and the RFS model.
Reclamation Long-Term Operational Model forColorado River System Reservoirs
Reclamation utilizes a long-term policy model for monthly analysisof operations extending in excess of 2 years commonly referred toas CRSS (U.S. Dept. of the Interior 1985; U.S. Dept. of the Interior1992). CRSS is developed within RiverWare, a flexible river basin
modeling software program (Zagona et al. 2001). In this study,a portion of the CRSS model is used to evaluate hydrologicconditions within the San Juan River Basin using a total of 224hydrologic scenarios (112 scenarios for each of the two hydro-logic models) encompassing three greenhouse gas emissionsscenarios: A1B, A2, and B1 (Intergovernmental Panel on ClimateChange 2000).
Previous study by Miller et al. (2011) indicates that there is littledifference between emission scenarios over the San Juan RiverBasin. The operation of the Navajo Reservoir within the CRSSis driven from a prescribed rule curve that generally seeks to fillthe reservoir in January through July and increase available capac-ity from August through December. Monthly releases, also set by arule curve, are increased to meet downstream water demands notmet by available water downstream of Navajo Dam.
CRSS Methodology
A modified version of Reclamation’s CRSS model is used to evalu-ate streamflow and reservoir operation effects within the San JuanRiver Basin under projected inflow scenarios developed from theVIC model and NWS RFS forced with projected climate conditionsfrom the temporally disaggregated BCSD data set. The CRSS
model is run at a monthly time step and, as such, may not explicitlycapture all important operational decisions made on a daily basiswithin the San Juan River Basin. However, the goal of the CRSSmodel is to adequately capture average, long-term operationswithin the San Juan River Basin for relevant policy decisions.Inflow to the San Juan River Basin consists of natural streamflowthat is primarily the result of snowmelt runoff, precipitation, andimports from the Dolores River Project.
Inflow into the Dolores River is required to run the modifiedCRSS over the San Juan River Basin. The RFS model providedby the CBRFC does not account for imported water into theSan Juan River Basin from the Dolores River Project. To accom-modate the lack of Dolores River streamflow in the RFS, projec-tions of streamflow along the Dolores River from the VIC modelwere used with both sets of natural inflow projections from the VICmodel and RFS. The other two inflow points into the model are theSan Juan River above Navajo (natural flow above Archuletta) andthe intervening natural flow between USGS gages on the San JuanRiver at Bluff, UT and Archuletta, NM (gains above Bluff). Theseinflow points were available as results from the RFS and VICmodels.
In 2007, the Secretary of the Interior signed the 2007Hydrologic Determination for Contracting of Water from Navajo
Fig. 2. (a) Annual total natural flow at Bluff for the historical record and for the raw and bias corrected verification run of VIC; (b) monthly averagetotal natural flow at Bluff UT, for the historic record (1950–1999) and for the raw and bias corrected verification run of VIC; (c) annual total naturalflow at Bluff for the historical record and for the raw and bias-corrected verification run of the RFS; (d) monthly average total natural flow at Bluff forthe historic record (1976–1999) and for the raw and bias-corrected verification run of the RFS
Reservoir for the Navajo-Gallup Water Supply Project (HydrologicDetermination) (Upper Colorado River Commission 2007). In theHydrologic Determination, it was determined that a likely watersupply exists within the Navajo Reservoir to service consumptiveuse for the Navajo Nation and NIIP through 2060. Projected con-sumptive uses and losses within the San Juan River Basin weremodeled as described in the Hydrologic Determination through2060. Consumptive use within the CRSS model was held constantat 2060 levels for the duration of the model run through 2099.
Results
Hydrologic Model Results
The annual gains above Archuletta, NM, gains above Bluff, UT,and the total San Juan Basin natural inflow (gains above Archuletta,NM, plus gains above Bluff, UT) were compared at the 10th, 50th,and 90th percentiles over each of the 112 model runs forced bystreamflow results from the VIC model or RFS. Projected naturalinflow above Archuletta, NM, derived through use of the RFS, isconsistently smaller at the median and 90th percentile than projec-tions derived from the VIC model. Differences also exist in thegains above Bluff, UT, between the VIC and RFS models, thoughintervening flow at the 90th percentile is similar between the twomodels (Fig. 3).
Over the span of the model run, the total average streamflowinput over the entire San Juan River Basin into the CRSS modelis approximately 14% higher from the VIC model than from theNWS RFS. These differences, particularly those reflected in thenatural flow above Archuleta, NM, where the average of annualstreamflows derived through the VIC model are approximately19% higher than those annual streamflows derived through theRFS, are primarily due to differences in assumptions of consump-tive use within the San Juan River Basin between Reclamation andthe CBRFC. In contrast, the average of annual streamflows above
Bluff, UT, is approximately 10% higher when derived through theuse of the VIC model.
Using the Kolmogorov-Smirnov test (KS-test) (Haan 1977), theempirical cumulative distributions of naturalized streamflow devel-oped through use of the VIC model and RFS are found to be differ-ent at the 99% confidence interval.
CRSS Results
The NWS defines the most probable projected streamflow as themedian flow result of an ensemble. Similarly, the maximum andminimum probable flow scenarios are those streamflow valuesat the 10 and 90% exceedance levels, respectively. The maximum,minimum, and most probable inflows into the Navajo Reservoirover the CRSS model run tend to decrease decadally, regardlessof whether or not the model was forced with data from the VICmodel or RFS. However, CRSS results exhibited a greater rangeof projected inflows when forced with streamflow developed bythe VIC model (Table 1).
TheNIIP requires 816.56 106 m3 (0.662 106 acre-ft) of storage inthe Navajo Reservoir during the agricultural season (April throughSeptember) for irrigation purposes; this storage corresponds with asurfacewater elevation of approximately 1,825.75m (5,990 f). Fig. 4illustrates the projected distribution of seasonal storage in theNavajoReservoir under climate change conditions. Over the 90-year modelrun period, median storage within the Navajo Reservoir is higherthan the critical elevation using both streamflow projections derivedusing the VIC model and the RFS; however, under streamflow con-ditions derived using the RFS, there is a higher risk that storagewithin the Navajo Reservoir would fall below the minimum storagelevel required for NIIP to operate.
The risk of the Navajo Reservoir falling below the NIIP criticalsurface water elevation during the agricultural season is projected toincrease in the future. This risk is increased by as much as 35%under model runs forced with streamflow projections developedfrom the NWS RFS; the risk increases to approximately 18% whenusing projections that are a result of the VIC model. As such, choiceof hydrologic model may influence critical decision variableswithin the San Juan River Basin.
Under projected streamflow conditions, the possibility of spills(water released from the Navajo Reservoir to avoid exceeding res-ervoir capacity) and shortage (water that is not able to be deliveredto downstream users) exists. Because the magnitude of streamflow,on average, is less throughout the San Juan River Basin when de-veloped using the RFS, the annual risk of spills from the NavajoReservoir is approximately 1% over the course of the model run.The average magnitude of spill when it does occur is approximately139 106 m3 (0.113 106 acre-ft). Under those model runs usingstreamflow developed by the VIC model, the average risk of a spillevent from the Navajo Reservoir over the projected period is ap-proximately 3% with an average magnitude of 222 106 m3
(0.180 106 acre-ft).The risk of shortage throughout the San Juan River Basin
increases throughout the model run projection (Fig. 5). Table 2illustrates that the average risk and magnitude of shortage withinthe San Juan River Basin increases throughout the decadal periodsencompassed over the model run projection. The risk of shortagewithin the San Juan River Basin is generally less above NavajoDam when using projections forced with streamflow developedusing the RFS than those projections developed using the VICmodel; the magnitude of shortage is less as well. This result iscounter intuitive because average annual flow developed from theVIC model is higher than average annual flow developed throughuse of the RFS. Water use demands and streamflow are defined at
Fig. 3. Streamflow projections are derived using projections of futureclimate from the WCRP CMIP3 data set to force the VIC model(dashed line) and RFS (solid line); the 10th, 50th, and 90th percentilesof 112 annual streamflow values within the San Juan River Basin areshown for (a) above Archuleta, NM, and (b) intervening flow aboveBluff, UT, which are the primary forcings for the CRSS
the monthly time step within CRSS; as such, average seasonalstreamflow generated by the VIC model during peak demandmonths is lower than average seasonal streamflow developedthrough use of the RFS. This result further underscores the impor-tance of understanding variations between hydrologic models thatgo beyond calibration and parameter validation procedures.
Below Navajo Dam, the risk of shortage within the San JuanRiver Basin is higher when using projections based on streamflowdeveloped through the RFS. The magnitude of shortage is approx-imately 45% higher (10 106 m3 or 0.01 106 acre-ft) when usingprojections forced with streamflow developed using the RFS thanthose developed using the VIC model. This substantial increase isdue, in part, to different assumptions regarding the routing and con-tribution of streamflow, as well as consumptive use, within the SanJuan River Basin below Navajo Dam. As such, these differences inassumptions, as well as differences in the hydrologic model, impacthow releases from the Navajo Reservoir are modeled.
Since 1970, average regulated streamflow at the San Juan Riverat the Bluff, UT, gauge has been approximately 1,865 106 m3
(1.512 106 acre-ft). This gauge is located just upstream of the con-fluence between the San Juan River and the Colorado River and
contributes to storage within Lake Powell. Average regulatedstreamflow is projected to decrease by an average of 35% (652106 m3 or 0.529 106 acre-ft) and 48% (899 106 m3 or 0.728 106
acre-ft) by using flows developed using the VIC and RFS models,respectively, over the course of the model run. This is reflected inthe seasonal magnitude and distribution of flows at Bluff, NM(Fig. 6). Streamflow is decreased consistently throughout the dec-adal periods covered by this projection.
Discussion
The decision between hydrologic models and associated output isan important consideration for any resource manager or stakeholderwhen trying to interpret potential risks and impacts to a watershedunder changing climate conditions. Often times, the availability andquality of data greatly influence the model decision. Here, tempo-rally disaggregated BCSD projections of precipitation and temper-ature data were used to derive projections of streamflow using thedistributed VIC model and the lumped NWS RFS. It is important tonote that variability and uncertainty in future climate projectionsexists [e.g., Brekke et al. (2004), Christensen and Lettenmaier(2007), Maurer and Duffy (2005), and Maurer (2007)]. As such,examining output from multiple GCMs may prove beneficial in
Table 1. Maximum, Minimum, and Most Probable Inflow Scenarios into the Navajo Reservoir
Note: The maximum, minimum, and most probable values represent the 10, 90, and 50% exceedance values, respectively.
Fig. 4. Projected seasonal distribution of storage within the NavajoReservoir over the model run [2010–2099; the critical storage forthe NIIP is indicated by the dashed line; median storage within thereservoir is typically above the critical elevation during the agriculturalseason (April through September)]
Fig. 5. (a) Risk; (b) magnitude of shortage events within the San JuanRiver Basin; bold lines indicate the risk and magnitude of shortageevents below Navajo Dam, while the thinner lines indicate the riskand magnitude of shortage above Navajo Dam
providing a broad sampling of possible future scenarios and somemeasure of the uncertainty expressed therein. By employing multi-ple hydrologic models, water resource managers are able to inves-tigate a broader range of outcomes, resulting in the potential formore robust risk assessments. Of further interest for future inves-tigation is the effect of projected climate choice on water manage-ment planning and decision making. Fig. 7 illustrates the broadrange of projected hydrology that may be developed from eachof the 112 projected climate scenarios utilized in this study. It isimportant to note that some of the largest flows are derived usingGCMs forced with the A1B emissions scenario, which may bedescribed as the more moderate scenario when compared withthe A2 and B1 scenarios. Furthermore, some of the driest projec-tions of future streamflow are derived under the B1 emissionsscenario, which typically exhibits a more optimistic view of futureclimate with a smaller increase in future temperatures. It may behypothesized that the choice of projected climate and GCM and,subsequently, how output from the GCM is manipulated (e.g., stat-istical or dynamical downscaling) has much more impact on futurewater supply analysis than the choice of hydrologic model.
In general, inflow into the Navajo Reservoir is projected to behigher using projections developed from the RFS; however, tribu-tary flow from the San Juan River Basin to the Colorado River is,
on average, 25% higher using projections developed from the VICmodel. Over the course of the 90-year projection, the projectedaverage total naturalized inflow into the San Juan River Basin isapproximately 15% higher (266 106 m3 or 0.215 106 acre-ft) usingstreamflow developed through use of the VIC model. These inflowsaffect the operation of and storage within the Navajo Reservoir,impacting water users within the San Juan River Basin, benefici-aries of the NIIP, and the efficiency with which Reclamation canmeet the requirements of the SJBRIP.
It is difficult to quantify the direct effects to operation of theNavajo Reservoir using monthly and decadal projections of stream-flow within the San Juan River Basin. The Navajo Reservoir isadaptively managed daily to meet water demands and remain incompliance with the SJBRIP. In this paper, a rule curve was usedin a modified version of Reclamation’s CRSS to approximate theoperation of the Navajo Reservoir at a monthly time step; as such,specific effects related to environmental and irrigation demandsmay not be fully captured. Despite this limitation, there is a pro-jected decrease of inflow to the San Juan River Basin and decreasedreleases from the Navajo Reservoir. Although choice of eitherhydrologic model does not qualitatively change projected effectsof climate change over the basin, there are quantitative impacts thatmay influence critical operational decisions for water resourcemanagers. Over the 2010–2039 period, projected risk of shortagebelow Navajo Dam is below 50% when using streamflows devel-oped through use of the VIC model; however, under streamflowscenarios developed through use of the RFS, the risk of shortageis 65%. In the event of shortage below the Navajo Reservoir, aver-age shortages are nearly 5 106 m3 (0.004 106 acre-ft) higher whenusing projections from the RFS than those projections developedthrough use of the VIC model.
It is important to study the effects of climate change to reservoirsin the Upper Colorado River Basin because their operation directlyimpacts storage at Lake Powell, which, in turn, may significantlyimpact storage at Lake Mead and water availability to ColoradoRiver water users in the Lower Colorado River Basin. Under chang-ing climate conditions, average streamflow from the San Juan RiverBasin may be reduced by up to 48%, or just under 900 106 m3
(0.73 106 acre-ft) of water. As such, regional climate change effectsover the San Juan River Basin and other headwater basins may havesignificant impacts on stakeholders throughout the ColoradoRiver Basin.
Understanding differences between applicability, data availabil-ity, methodology, and calibration is important, particularly over theColorado River Basin. The RFS model relies on assumptions thatmay be more applicable to short-term forecasting of water resour-ces. Consumptive use within the San Juan River Basin is assumedto be relatively constant, and the RFS is calibrated and dependenton a shorter hydroclimatic record; thus, the applicability of thismodel to investigate long-term effects of climate change to reser-voir operations may be limited. Thus, a further understandingthe various nontechnical differences between hydrologic models,
Table 2. Summary of the Risk and Magnitude of Shortage within the San Juan River Basin above and below the Navajo Reservoir
Risk and magnitude of shortages within the San Juan River Basin (106 m3)
Fig. 6. Seasonal distribution of streamflow at the San Juan River at theBluff, UT, gage; historical observations of streamflow at the gage areover the 1970–2009 record and streamflow projections from the NWSRFS and VIC model cover the 2010–2099 period
agency assumptions of consumptive use and water diversions, andreconciliation of those differences is vital to understanding theeffects of hydrologic model choice, particularly in climate changeanalysis. Because the VIC model is calibrated to a longer-term,naturalized flow record, it may be inherently better able to capturemore of the climate variability that is so prevalent in the ColoradoRiver Basin for long-term climate change studies.
Although there is significant discrepancy between the magni-tude and risks associated with results between the two hydrologicmodels presented here, the results underscore the need for waterresource managers and water users to continue to improve theefficient use of resources to meet consumptive use, environmental,and other important demands within the San Juan River Basin andthe entire Colorado River Basin.
Acknowledgments
The authors would like to thank the U.S. Department of the Interior,Bureau of Reclamation, Lower and Upper Colorado Regions fortheir continued support and funding of this work. Additional infor-mation regarding this study, and other Reclamation studies, can beobtained by contacting the corresponding author. The authors alsoacknowledge the modeling groups, the Program for Climate ModelDiagnosis and Intercomparison (PCMDI) and the WCRP’s Work-ing Group on Coupled Modeling (WGCM) for their roles in mak-ing available the WCRP CMIP3 multi model dataset. Support ofthis data set is provided by the Office of Science, U.S. Department
of Energy. The authors would also like to thank Andy Wood withthe CBRFC for making available the temporally disaggregatedBCSD used in the development of this analysis. The researchat University of Nevada, Las Vegas is supported by grants NSFEPS-0814372, NOAA NA07AR4310228, DOE DE-FG02-08ER64709, and DOE DE-EE-0000716.
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