A METHOD TO CONSIDER WHETHER DAMS MITIGATE

CLIMATE CHANGE EFFECTS ON STREAM TEMPERATURES1

Sarah E. Null, Scott T. Ligare, and Joshua H. Viers2

ABSTRACT: This article provides a method for examining mesoscale water quality objectives downstream ofdams with anticipated climate change using a multimodel approach. Coldwater habitat for species such as troutand salmon has been reduced by water regulation, dam building, and land use change that alter stream temper-atures. Climate change is an additional threat. Changing hydroclimatic conditions will likely impact water tem-peratures below dams and affect downstream ecology. We model reservoir thermal dynamics and releaseoperations (assuming that operations remain unchanged through time) of hypothetical reservoirs of differentsizes, elevations, and latitudes with climate-forced inflow hydrologies to examine the potential to manage watertemperatures for coldwater habitat. All models are one dimensional and operate on a weekly timestep. Resultsare presented as water temperature change from the historical time period and indicate that reservoirs releasewater that is cooler than upstream conditions, although the absolute temperatures of reaches below dams warmwith climate change. Stream temperatures are sensitive to changes in reservoir volume, elevation, and latitude.Our approach is presented as a proof of concept study to evaluate reservoir regulation effects on streamtemperatures and coldwater habitat with climate change.

(KEY TERMS: water temperature; water management; climate change; regulated rivers; reservoirs; coldwaterhabitat; Sierra Nevada.)

Null, Sarah E., Scott T. Ligare, and Joshua H. Viers, 2013. A Method to Consider Whether Dams MitigateClimate Change Effects on Stream Temperatures. Journal of the American Water Resources Association(JAWRA) 49(6): 1456-1472. DOI: 10.1111/jawr.12102

INTRODUCTION AND RATIONALE

Stream temperature is an instrumental character-istic of lotic systems, directly influencing dissolvedoxygen levels, nutrient cycling, chemical reactionrates, productivity, and mortality in aquatic ecosys-tems (Poole and Berman, 2001). Stream temperatureis influenced by climate, making it particularly vul-nerable to climate change (Mohseni and Stefan,1999). It is driven by heat exchange with the

atmosphere and streambed, as well as source temper-atures (i.e., groundwater, snowmelt, overland flow,and precipitation). Long-term data analysis hasshown that stream temperatures have alreadyincreased as air temperatures have risen in recentdecades (Hari et al., 2006; Kaushal et al., 2011), andmodeling studies suggest that stream temperatureswill increase further with climate warming, poten-tially causing habitat reduction for coldwater species(Jager et al., 1999; Battin et al., 2007; Null et al.,2013).

1Paper No. JAWRA-12-0146-P of the Journal of the American Water Resources Association (JAWRA). Received June 14, 2012; acceptedApril 29, 2013. © 2013 American Water Resources Association. Discussions are open until six months from print publication.

2Respectively, Assistant Professor (Null), Department of Watershed Sciences, Utah State University, 5210 Old Main Hill, NR 201, Logan,Utah 84321; Water Resources Control Engineer (Ligare), California State Water Resources Control Board, Sacramento, California 95814;and Associate Director (Viers), Center for Watershed Sciences, University of California, Davis, Davis, California 95616 (E-Mail/Null: sarah.null@usu.edu).

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A growing literature links climate change specifi-cally with water quality and quantity for salmonid spe-cies (salmon and trout), but does not always considerregulated systems. Research in unregulated systemsindicates that stream warming could reduce the totalamount of coldwater habitat (Eaton and Scheller, 1996;Bogan et al., 2003; Tung et al., 2006; Battin et al.,2007; Isaak et al., 2010; Null et al., 2013) and shift it tohigher elevations or latitudes (Jager et al., 1999; Hariet al., 2006; Isaak et al., 2010; Null et al., 2013), alterthe distribution and abundance of coldwater species(Gooseff et al., 2005), or introduce pathways for inva-sive species (Rahel and Olden, 2008). Most climatechange research in regulated systems focuses onimpacts to people, such as water supply, hydropowergeneration, and flood control (Hamlet and Lettenmaier,1999; Barnett et al., 2005; Medell�ın-Azuara et al.,2008; Madani and Lund, 2010; Das et al., 2011).

It is not well understood the extent to which damscan provide coldwater releases to maintain coldwaterhabitat in coming decades or how climate changemay impact release temperatures. Reservoir releasesmay present an opportunity to manage water temper-ature and provide suitable coldwater habitat for fishand wildlife, particularly by releasing water from thecoldwater pool of thermally stratified reservoirs. Res-ervoir releases provide new boundary conditions fordownstream reaches, with diminishing influence asdistance from the dam increases. It is possible thatdams could mitigate some of the stream temperatureimpacts of climate change, particularly where reser-voir releases are colder than unregulated conditionsor where winter water is stored in the coldwater poolof deep reservoirs. A handful of articles focus onpotential climate warming impacts to stream temper-atures in regulated systems (Sinokrot et al., 1995;Johnson et al., 2004; Gooseff et al., 2005; Yates et al.,2008). Sinokrot et al. (1995) found that climatechange effects regulated stream temperatures more ifreservoirs release from the surface rather than thedeeper coldwater pool. A study of the Blue Mesa Res-ervoir on the Colorado River showed that climateplays the largest role in reservoir thermal structureand that a warmer climate may reduce the volume ofthe hypolimnion (Johnson et al., 2004). Yates et al.(2008) modeled California’s Shasta Reservoir anddownstream Sacramento River, and determined thedam could provide coldwater to offset the effects ofclimate change and maintain downstream habitat fornative Chinook salmon. Gooseff et al. (2005) foundthat stream temperatures downstream of a shallowreservoir in Montana’s Lower Madison River areexpected to increase from climate change and thatthey are driven by daytime warming.

The objective of this study was to develop amethod to evaluate the potential of reservoirs for

mitigating climate change effects on stream tempera-tures at a regional scale. Reservoirs of varying size,latitude, and elevation in California’s west-slopeSierra Nevada mountain region are used for thisproof of concept study. This research begins to quan-tify the extent to which dams may provide thermalbenefit under “normal operations” — a generic oper-ating rule curve that does not change operations toexplicitly manage for coldwater. This research isnovel because it explores thermal management at themesoscale, rather than analyze temperatures on acase-by-case basis for individual reservoirs. It isintended to guide operations and help researchersand water managers who are relied on to providewater for multiple and competing uses.

This study begins with a description of the SierraNevada study area, including a discussion of currentwater development. We explain our modelingapproach, including hydroclimate input data,the Water Evaluation and Planning (WEAP) rainfall-runoff model used to simulate water operations; theWater Quality for River-Reservoir Systems (WQRRS)water quality model used to estimate reservoir temper-atures; and the Regional equilibrium stream TEMPer-ature (RTEMP) model. We discuss reservoir releasetemperatures with climate change, compare unregu-lated and regulated stream temperatures, analyze theeffect of reservoirs of different sizes, elevations, andlatitudes on stream temperature longitudinal profiles,and end with a discussion of data uncertainty. Overall,the approach described here provides a method fordam operators or other stakeholders to incorporatewater quality effects of climate change with water sup-ply, hydropower, and flood control impacts for a morecomplete understanding of climate change impacts ofwater regulation. Aquatic and riparian habitats areoften directly or indirectly managed by water opera-tions and reservoir releases which alter downstreamwater quantity, quality, and timing. As water is man-aged more tightly in a future with climate change, thistype of research will help to evaluate how reservoirreleases may benefit ecosystems.

BACKGROUND

Study Area

California’s west-slope Sierra Nevada rivers flowwestward from the crest of the Sierra Nevada throughthe confluence with the Sacramento-San Joaquin Deltaand to the Pacific Ocean (Figure 1). The Sierra Nevadamountain region has a Mediterranean-montane cli-mate with a dry season roughly from May to October

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and a wet season from November to April. Historically,snowline was about 1,000 meters (m). Precipitationaveraged about 100 cm/yr for the region, but variedgreatly with latitude, elevation, and local weather pat-terns. Precipitation was greatest in the northernwatersheds and the highest elevations of the southernSierra Nevada. Typically, snowpack persisted late intosummer.

Modeling suggests that unregulated stream tempera-tures are anticipated to increase approximately 1.6°Cfor each 2°C rise in air temperature in this region (sim-ulated with uniform air temperature increases of 2, 4,and 6°C as a sensitivity analysis of stream temperatureresponse to climate warming) (Null et al., 2013) (Fig-ure 2). However, Sierra Nevada rivers have been exten-sively developed for water supply, hydropowergeneration, flood protection, and recreation, which hasfundamentally altered the natural flow and tempera-

ture regime, threatening the health and integrity offreshwater ecosystems (Poff, 2009; Olden and Naiman,2010). West-slope watersheds from the Featherwatershed in the north to the Kern watershed in thesouth have about 24,590 million cubic meters (mcm) ofwater storage for all dams greater than 1.2 mcm(1 thousand acre feet [taf]), and approximately 8,751MW of online hydropower capacity (Null et al., 2010a).Infrastructure consists of a complex network of dams,diversions, canals, powerhouses, and transmission lines.

In general, the distribution of reservoirs is approxi-mately equal by elevation, although storage capacityis much greater at lower elevations (Table 1).Roughly 75% of California’s in-state hydropower isproduced from more than 150 dams in the SierraNevada Mountains at higher elevations (Aspen Envi-ronmental and M.Cubed, 2005), which are mostlyprivately owned and operated solely for hydropowerrevenues (Vicuna et al., 2008). Winter rain and

FIGURE 1. Study Watersheds and Topography (Null et al., 2010a).

FIGURE 2. Average Annual Number of Weeks UnregulatedStream Temperatures Exceed 24°C (lethal threshold for Chinooksalmon and steelhead trout (Eaton and Scheller, 1996)) with Incre-mental Uniform 2°C Air Temperature Increases (T0, T2, T4, andT6 represent climate warming of 0, 2, 4, and 6°C, respectively)(Null et al., 2013).

TABLE 1. Major Sierra Nevada Reservoirs by Elevation, Latitude, and Average Volume (mcm).

Elevation Group

Northern (Feather,Yuba, Bear, American,

Cosumnes)

Central (Mokelumne,Calaveras, Stanislaus,Tuolumne, Merced)

Southern (SanJoaquin, Kings,

Kaweah, Tule, Kern) Total

Very high (>1,600 m) n = 12avg. vol. = 50

n = 4avg. vol. = 82

n = 6avg. vol. = 137

n = 22avg. vol. = 80

High (800-1,600 m) n = 14avg. vol. = 206

n = 7avg. vol. = 171

n = 2avg. vol. = 102

n = 23avg. vol. = 186

Mid (<800 m) n = 9avg. vol. = 800

n = 6avg. vol. = 1,313

n = 7avg. vol. = 412

n = 22avg. vol. = 813

Total n = 35avg. vol. = 305

n = 17avg. vol. = 553

n = 15avg. vol. = 261

n = 67avg. vol. = 357

Note: mcm, million cubic meters.

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spring snowmelt are stored in relatively small reser-voirs that provide within-year storage for generationprimarily during summer. The remaining 25% of Cal-ifornia’s in-state hydropower is produced in large, lowelevation, multiuse projects operated for water sup-ply, hydropower, and flood protection, which areowned and operated by the state and federal govern-ments (Aspen Environmental and M.Cubed, 2005).

METHODS

Our approach was to model a simple, hypotheticalwatershed that has many of the physical characteris-tics found in watersheds of the Sierra Nevada suchas hydropower, water supply, and flood control infra-structure, as well as unregulated tributaries. Thismethod required a series of models that, when linkedvia parameterization and sequencing, can be appliedto a large spatial area such as California’s SierraNevada and have the ability to represent multipleyears and climate scenarios quickly.

Infrastructure Representation

Our hypothetical watershed had three main forks,with a total drainage area of 2,341 km2 (Figure 3).Elevation ranges 1,200 m with three reservoirs inseries. The middle fork is the largest of the threeforks and has two hydropower projects, which consistof a reservoir and a diversion to a downstream hydro-power plant. Reservoirs are 58.1 mcm (approximatelythe size of Ice House Reservoir in the Americanwatershed) and 86.9 mcm (about the size of BowmanLake in the Yuba watershed), respectively (Table 2).The north fork and the south fork are undammedand have a natural flow regime.

The lowest reservoir is below the confluence of allthree forks near the bottom of the watershedrepresenting a multipurpose “rim” dam, managed forwater supply, hydropower, and flood control. The sizeof rim reservoirs in the Sierra Nevada varies from86.3 mcm (Englebright on the Yuba River) to over

4,363 mcm (Oroville on the Feather River). We mod-eled a reservoir approximately the size of FolsomLake to assess climate change effects on rim dams(Table 2). Flood control operations were developed bysimplifying the U.S. Army Corps of Engineers Folsomflood control curve. The top of conservation pool wasdrawn down to 500 mcm from week 41 to week 46and held constant for the entire flood season regard-less of storage space available upstream. From week13 to week 21 the flood control space was reduced tozero for the remainder of the year. We representedwater supply operations in the lower reservoir withwater demand that peaks in the summer and is at aminimum during winter months, based on actual waterdemand (Department of Water Resources, 2009).

We modeled hydropower operations by requiring aminimum release from the reservoir for each week,which was based on average weekly hydropowerreleases from hydropower plants in the SierraNevada (Madani and Lund, 2010). This methodassumes that hydropower demand does not changewith water year type or climate.

To assess the effects of climate change on streamtemperatures over a spectrum of latitudes, the entiretheoretical watershed was shifted from the central

FIGURE 3. Hypothetical Study Watershed with ModeledInfrastructure.

TABLE 2. Physical Characteristics of Modeled Reservoirs.

ReservoirElevation

ReservoirSize

Elevation(m)

StorageCapacity (mcm)

HydropowerCapacity (MW)

TurbineCapacity (m3/s)

Upper S 1,615 58.1 12 9.0Middle M 1,291 86.9 65 25.5Lower L 300 1,233.0 200 86.8

Note: mcm, million cubic meters.

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Sierra Nevada (in the Stanislaus River) about200 km north to the approximate location of theFeather watershed and about 100 km south to theapproximate location of the San Joaquin watershed(Figure 4). Watershed size, shape, and infrastructureremained constant and only climatic inputs andhydrology were changed with latitude.

Input Data and Modeling Approach

We used two general circulation models (GCMs)that represent extremes for possible precipitation andtemperature, MIROC32med and NCARPCM1, MI-ROC32med is warmer and drier whereas NCARPCM1is cooler and wetter (Cayan et al., 2009). Both GCMsused the A2 emissions scenario, which assumes moder-ately severe climate change and continuously increas-ing population growth (Weare, 2009). GCM output wasdownscaled to 1/8° using the bias correction and spa-tial downscaling method described in Maurer et al.(2010). Climate input data drove the Variable Infiltra-tion Capacity (VIC) rainfall-runoff model to producesurface runoff, base flow, evapotranspiration, and soilmoisture on a 1/8° spatially gridded format (Maurer,2007). These data were provided by the CaliforniaNevada Applications Program/California ClimateChange Center (Table 3).

WEAP, WQRRS, and RTEMP were run sequen-tially, and are described below (see modelingsequence in right side of Figure 4). A weekly timestepwas used for all models. This study required 48 modelruns, representing two GCMs, two management sce-narios for regulated and unregulated conditions,three latitudes, and four 20-year time periods rangingfrom 1970 to 2090, which were labeled as historical(htp), near-term (ntp), mid-term (mtp), and far-term(ftp) time periods (Table 4).

Reservoir Operations and Outflows — WaterEvaluation and Planning. WEAP is a water man-agement and hydrology software package developedby the Stockholm Environment Institute. Here,hydrology was modeled with the VIC model by theCalifornia Nevada Applications Program/CaliforniaClimate Change Center and WEAP was used tomodel water management including infrastructure,water demands, and allocation priorities (Yates et al.,2005). It operates on a weekly timestep with spatialrepresentation of 250-m elevation bands (Younget al., 2009). We placed the highest allocation priorityon water supply, second on hydropower generation,and third on reservoir storage. WEAP output datawere passed to RTEMP and WQRRS (Figure 4). SeeYates et al. (2005) for WEAP governing equationsand additional model detail, Young et al. (2009) for adescription of unregulated Sierra Nevada flow model-ing, Null et al. (2010a) for hydrologic sensitivity toclimate warming in California’s Sierra Nevada, andMehta et al. (2011) for climate change impacts onSierra Nevada hydropower production.

Reservoir Water Quality Modeling — Water Qualityfor Reservoir-River Systems. Reservoir temperatureswere simulated with WQRRS, a one-dimensionalriver and reservoir model developed by the U.S. ArmyCorps of Engineers (USACE-HEC, 1986). WQRRS isa physically-based, finite difference model based onthe principles of conservation of heat and mass. Themodel operates on a daily timestep, although we usedaggregated weekly input and output data (we inputaverage weekly inflow, stream temperature, andweather data and used average weekly water temper-ature output). We chose WQRRS because of quickrun times (20 years were simulated in seconds). Forthis study, we used only the reservoir simulationcomponent (not the river simulation component sothat unregulated stream temperatures previouslymodeled with RTEMP were comparable). Water tem-perature was the only water quality parameter mod-eled and was estimated using the heat budgetmethod (USACE-HEC, 1986).

WQRRS has been used extensively through thepast three decades to model water quality in lakes

FIGURE 4. West-Slope Watersheds of the Sierra NevadaShowing Locations of Hypothetical Watersheds and Reservoirs

(left side) and Model Run Flow chart (right side).

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and reservoirs, with recent publicationsreviewed here. It was used to aid decision making byevaluating management alternatives for sustainableecosystem management of Turkish coastal lagoons(Gonenc et al., 1997), and to aid conflict resolutionover water allocation by modeling Iran’s Karkhehriver-reservoir system (Karamouz et al., 2008). Deasand Orlob (1999) simulated water quantity and qual-ity for restoration of anadromous fisheries below IronGate Dam on the Klamath River. Lopes et al. (2003)used it to simulate dissolved oxygen and water tem-perature in Portugal’s Aguieira Reservoir, and Meyeret al. (1996) altered meteorologic and hydrologicinput conditions to estimate water quality effects onShasta Reservoir and part of the Sacramento Riverfrom a doubling of CO2.

Water temperature and other water quality param-eters are simulated using the one-dimensional formof the advection-dispersion equation:

V@C

@tþ DxQx

@C

@x¼ DxAxDc

@2C

@x2þQiCi �QoC� VS ð1Þ

where C is thermal energy (kcal), V is volume (m3),t is time (s), x is vertical distance in the reservoir(m), Qx is advective flow (m3/s), Ax is surface area(m2), Dc is the effective diffusion coefficient (m2/s), Qi

is lateral inflow (m3/s), Ci is inflow thermal energy(kcal), Qo is lateral outflow (m3/s), and S are sourcesand sinks (kcal/s). Internal transport of heat andmass occurs only in the vertical direction throughadvection and effective diffusion mechanisms, withwater well mixed laterally and longitudinally. Weassumed all reservoir inflow occurs at the upstreamend of the reservoir, making the rate of advectionslower than if the inflow occurred closer to the outlet.Effective diffusion between layers is based on temper-ature and includes molecular and turbulent diffusionas well as convective mixing based on density gradi-ents. One-dimensional models such as WQRRS workwell for small to large reservoirs, but should not beused for shallow reservoirs or reservoirs with smallresidence times (i.e., vertically mixed systemsthat would be better represented with a river model)(USACE-HEC, 1986).

Reservoirs are represented as a series of one-dimensional layers, with user-defined surface area,thickness, and volume. For this application, layerthickness was 1 m. Inflows are instantaneously dis-persed throughout the reservoir layer of similar den-sity. Outflows are modeled using the selectivewithdrawal allocation method developed by the U.S.Army Engineer Waterways Experiment Station (WESmethod) (USACE-HEC, 1986) to describe the verticallimits of the withdrawal zone and the vertical velocitydistribution within the zone. For additional model

TABLE 3. Model Input Data and Source. RTEMP Input from Null et al. (2013), WEAP Input from Young et al. (2009).

Model Input Source

VIC GCM (MIROC32med and NCARPCM1–A2SRES) boundary conditions

California Climate Change Center

RTEMP Meteorology VIC (cloud cover user defined)Hydrology (surface runoff, snow accumulation,snowmelt, groundwater)

VIC

Topography USGS 10 m digital elevation modelReservoir outflow WEAPReservoir release water temperature WQRRS

WEAP Inflow hydrology VICReservoir operational rules User defined

WQRRS Meteorology VIC (wet bulb temperature calculated, cloudcover user defined)

Geometry User definedFlow (inflow, outflow, initial storage) WEAPInflow water temperature RTEMP

Note: RTEMP, Regional equilibrium stream TEMPerature; WEAP, Water Evaluation and Planning; VIC, Variable Infiltration Capacity;WQRRS, Water Quality for River-Reservoir Systems.

TABLE 4. Model Run Matrix.

Variable Model Runs Count

Climate models A2-MIROC32medA2-NCARPCM1

2

Time periods htp (1970-1990)ntp (2010-2030)mtp (2040-2060)ftp (2070-2090)

4

Latitudes NorthernCentralSouthern

3

Management1 RegulatedUnregulated

2

Total model runs 48

Notes: htp, historical period; ntp, near-term period; mtp, mid-termperiod; ftp, far-term period.1Reservoir operation identical for all regulated runs (generic oper-ating rules).

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detail, see USACE-HEC (1986). Here, we aimed torepresent reservoir outlet structures in a genericway, and modeled withdrawal outlets as a small(<1 m3/s) withdrawal structure near the bed of thereservoir and a second outlet near the top of the cold-water pool for the remainder of outflow (modeled withWEAP). Spill events occur from the reservoir surface.In reality, there is quite a bit of variability in outletstructure design. The quantity and quality of reser-voir releases form the boundary conditions for down-stream river temperature models. See Table 3 for alist of input data and sources used for each reservoirmodel. Results from WEAP and WQRRS are passedto RTEMP to estimate stream temperatures, and themodels are run sequentially (Figure 4).

Stream Temperature Model — Regional Equi-librium Temperature Model. Stream temperatureswere estimated with RTEMP, a one-dimensional equi-librium temperature model developed by WatercourseEngineering, Inc. (Null et al., 2013). We used a weeklytimestep and spatial resolution of 250-m vertical eleva-tion bands for streams (the same resolution as WEAP).RTEMP is a physically-based model, which uses mete-orology, hydrology, boundary conditions, and streamlength (Table 3) to estimate stream temperaturesusing equilibrium temperature theory (Edinger et al.,1968).

Stream temperatures are estimated using a simpli-fied form of the advection-dispersion equation:

@Tw

@t¼ Hnet

Cpqdð2Þ

where Tw is water temperature (°C), t is time (wk),Hnet is net heat exchange at both the bed- and air-water interfaces (W/m2), Cp is the specific heat ofwater (4,185 J/kg/°C), q is water density (1,000 kg/m3),and d is water depth (m). Equation (2) assumes thatrivers are advection dominated and that changes inlongitudinal stream temperatures are driven by mete-orological conditions, which alter stream tempera-tures with respect to time. It further assumes arectangular channel where water surface is the samewidth as the streambed. For a full derivation andadditional model detail, see Null et al. (2013). Equa-tion (2) produces a time series of dynamic streamtemperatures where water seeks equilibrium temper-ature, but is constrained by boundary conditions,travel time, and river geometry.

Stream temperatures are estimated separately fortributaries and mainstem rivers. This allows attenua-tion of tributaries to be assessed. Mainstem and trib-utary channels mix at the bottom of each reach usinga mass balance approach. In addition to boundaryconditions for flow and stream temperature obtained

from WEAP and WQRRS, we assumed that snowmeltwas 1°C to account for slight heating during overlandflow from snowpatch to stream, precipitation temper-ature was the same as air temperature, and ground-water temperature was 7.5°C, which is the meanannual air temperature for the west-slope SierraNevada at 2,500 m.

Equilibrium temperature is useful because heatflux at the water surface can be represented withonly meteorological input variables, and it can beapplied on a regional scale. RTEMP has previouslybeen used to assess unregulated stream temperaturesensitivity to climate warming in California’s west-slope Sierra Nevada, with an emphasis on impacts tocoldwater habitat for anadromous salmon species(Null et al., 2013). Other equilibrium temperaturestudies exist. Notable examples include assessing theeffects of climate change by examining the relation-ship between stream and air temperatures (Mohseniand Stefan, 1999), predicting stream temperatures inCanada’s Miramichi River (Caissie et al., 2005), mod-eling Colorado River temperatures downstream ofGlen Canyon Dam (Wright et al., 2009), and quanti-fying the effects of stream discharge on summer rivertemperature (Gu et al., 1998).

Model Calibration and Testing

WEAP and RTEMP models were previously cali-brated for the 15 west-slope Sierra Nevada water-sheds with natural flow hydrologic conditions (Younget al., 2009; Null et al., 2013). Reservoir water tem-peratures, modeled with WQRRS, were calibratedusing measured temperature profiles from IcehouseReservoir, Bowman Reservoir, and Folsom Reservoirfor the upper, middle, and lower reservoirs, respec-tively. Calibration dates differ between modeled res-ervoirs depending on when measured data wereavailable. Most stream and reservoir temperaturestudies in the Sierra Nevada are part of FederalEnergy Regulatory Commission (FERC) relicensing,and therefore are limited to short-term collectionefforts.

The upper reservoir calibration was completedusing 13 thermal profiles. Daily average streaminflow and outflow temperatures from October 1,2000 to October 19, 2004, were collected by Sacra-mento Municipal Utilities District (SMUD, unpub-lished data) and observed streamflow measurementswere from the U.S. Geological Survey (USGS) (USGS,2012). The middle reservoir was calibrated with twoyears of measured data from Nevada Irrigation Dis-trict’s (NID) Yuba-Bear project (NID, unpublisheddata). Inflow and outflow temperature data included15-min temperature observations from June 3, 2008

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to October 10, 2008, and June 12, 2009 to October 16,2009. Reservoir thermal profile measurements werecollected by NID in July, August, September, andOctober of 2008 and 2009. Inflow discharge was esti-mated with USGS measured daily outflows throughthe Bowman-Spaulding Canal and Canyon Creekbelow Spaulding (USGS, 2012). The lower reservoirwas calibrated with Folsom Lake inflow and outflow,as well as stream temperature data from the AmericanRiver below Folsom Reservoir from September 30,2000 to April 10, 2011 (CDEC, 2011). No thermal pro-files for Folsom Reservoir were available for calibra-tion. Evaporation coefficients and radiation absorptioncoefficients were adjusted so modeled profiles and out-let temperatures reasonably matched observed data,and represent observed stratification and mixingwithin each reservoir.

RESULTS

Overall, vertical temperature profiles within reser-voirs matched observations well, indicating that mod-els capture the timing and depth of thermalstratification (see Null et al., 2012 for figures of mod-eled and observed vertical temperature profiles).Mean bias, mean absolute error (MAE), and rootmean square error (RMSE) were calculated for reser-voir release temperatures to evaluate WQRRS perfor-mance for the upper, middle (measured data fromtwo similar sites were compared), and lower reser-voirs. MAE was always less than 2.4°C and averaged1.8°C for all sites with measured data. Calibrationstatistics are summarized in Table 5. Reservoirrelease temperatures are important for stream tem-peratures because they provide the boundary condi-tions for downstream temperatures and thus candrive thermal response.

WEAP models were previously calibrated for the15 west-slope Sierra Nevada watersheds with natural

flow hydrologic conditions using monthly natural flowestimates at watershed outlets, and snow waterequivalent measurements at upstream sites withineach watershed (Young et al., 2009). Overall, WEAPrepresents flow hydrographs well at the watershedscale, and mean bias between simulated and mea-sured natural flow at watershed outlets was �1%,with a range �6 to 2%. RTEMP models were also pre-viously calibrated for unregulated conditions and aredescribed in detail in Null et al. (2013). Simulatedstream temperatures fit measured data well and thetiming of seasonal maximum temperatures was con-sistent between modeled and observed data. MAEwas within 1.2°C of measured maximum or minimumtemperatures, with a mean of 0.4°C.

Regulated Stream Temperatures in a ChangingClimate

Study findings focus on reservoir release tempera-tures with climate change for reservoirs of differentsizes and elevations. Results also compare regulatedstream temperatures with unregulated stream tem-peratures to highlight how climate change may affectregulated systems differently than unregulated sys-tems. We discuss study results by focusing on thecentral latitude, although the northern, central, andsouthern latitudes are compared with longitudinalstream temperature profiles at the end of this section.Major findings from this work illustrate the utility ofour approach as a proof of concept study. Overall,modeled stream temperatures with anticipated cli-mate change representing ftp conditions are coolerthan current observed stream temperatures. Wediscuss this and provide a brief analysis of input dataat the end of this section.

Reservoir Release Temperatures. Modelingsuggests that average reservoir release temperatureswill be resilient to climate change through winter(Figure 5). Reservoir release temperatures warm

TABLE 5. Calibration Model Fit Statistics by Reservoir.

Measured Location

Upper Middle Lower

Icehouse ReleaseBowman-Spaulding

CanalCanyon Creek below

SpauldingFolsomRelease

n, count 187 40 66 547Mean bias, °C �0.1 0.7 0.9 �0.7MAE, °C 1.1 1.9 2.4 1.6RMSE, °C 1.5 2.3 3.0 2.0Calibration date,month/date/year

10/1/00-10/19/04 6/3/08-10/10/08, 6/12/09-10/16/09 6/3/08-10/10/08, 6/12/09-10/16/09 9/30/00-4/10/11

Note: MAE, mean absolute error; RMSE, root mean square error.

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from climate change beginning in early May. Spring-time warming was particularly evident at the upperand middle reservoirs (Figures 5a and 5b), indicatingthat the effects of climate change on reservoirreleases may be more severe at higher elevations.Simulations using MIROC32med input climate datawere warmer than those using NCARPCM1 data(MIROC32med predicts a warmer and drier futurethan NCARPCM1). In fact, release temperatures forNCARPCM1 ftp were similar to release temperaturesfor MIROC32med ntp (Figure 5). This is indicative ofuncertainty in climate modeling, such as representa-tion of physical processes and sensitivity to green-house gas forcings.

Reservoir release temperatures were cooler for themiddle reservoir than the upper reservoir, reflectingmodel calibration and reservoir depth. The upper andmiddle reservoirs were calibrated to Ice House andBowman Reservoirs, respectively. Bowman Reservoirhad exceptionally cold release temperatures (muchcolder than Ice House Reservoir), a result of the

combination of outlet works infrastructure, reservoiroperations, depth, and inflow temperatures. Becausethe middle reservoir is calibrated to Bowman Reser-voir, some of the above factors are implicitly includedin this analysis. In addition, the modeled middle res-ervoir (Figure 5b) is deeper than the upper reservoir(Figure 5a), potentially affecting the volume of thecold-water pool, with cooler release temperatures dur-ing summer. The middle reservoir illustrates theimportance of parameterizing and calibrating modelsfor specific systems or regions when using thisapproach.

On average, the warmest water is released inAugust to early September for the upper reservoir,late July to early August for the middle reservoir,and early October for the large, lower reservoir (Fig-ure 5). However, there is considerable variabilitybetween years that is not depicted in the annuallyaveraged figures. For the upper reservoir, the timingof the warmest releases generally became more con-sistent with more severe climate change (mtp and

FIGURE 5. Average Weekly Central Latitude MIROC32med and NCARPCM1 Reservoir ReleaseTemperatures by Time Period for the (a) Upper, (b) Middle, and (c) Lower Reservoirs

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ftp). For all reservoirs, the warmest release tempera-tures coincided with extended periods of low reservoirstorage which forced mixing of stratified waterswithin the reservoir, or less commonly, with spillevents. The upper reservoir typically had less storedwater earlier in the year than the large, lower reser-voir. For the lower reservoir, low reservoir levelsmost commonly occurred in early fall, which is consis-tent with observed storage levels in large rim dams(CDEC, 2011).

Effects of Reservoirs on Stream Tempera-tures. For unregulated conditions, modeling sug-gests that stream temperatures warm with moresevere climate change, represented as a farther

outlook into the future (Figure 6, first and third col-umns, time progresses from the htp, ntp, mtp, andftp in the downward direction). For all unregulatedruns, the middle river that reaches from approxi-mately 150-250 river kilometer (RKM) is the warm-est, before tributaries subsequently join themainstem with cooler temperatures and higher flowsthat moderate mainstem temperatures. Toward theend of the 21st Century, modeling indicates thatstreams not only become warmer, but that the aver-age duration with warmer temperatures is extendedas well (x-axes in Figure 6).

Unregulated winter stream temperatures becomeapproximately 2°C warmer as well. For MIROC32medunregulated simulations, maximum average weekly

FIGURE 6. Modeled Average Weekly Stream Temperatures for the Central Latitudewith Regulated and Unregulated Conditions for Both GCMs and All Time Periods.

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stream temperatures are 13.1, 14.4, 15.8, and 18.2°Cfor htp, ntp, mtp, and ftp, respectively. For unregu-lated NCARPCM1 runs, stream temperatures are13.2, 13.2, 14.3, and 14.8°C for htp, ntp, mtp, and ftp,respectively. The warmer stream temperatures in theMIROC32med runs are largely driven by air tempera-tures that increase at double the rate of NCARPCM1air temperatures for the ntp, mtp, and ftp.

The relatively cool reservoir releases of the upperand middle reservoirs (Figure 5) provide cooler streamtemperatures during summer and/or a shorter dura-tion of warm stream temperatures than occur withunregulated conditions. However, the longitudinallength of cooler stream temperatures below dams isvariable (Figure 6 — second and fourth columns). Forexample, cooler temperatures below the middle reser-voir vary by approximately 10-90 RKM depending onGCM input data and modeled time period. There is noclear trend as to whether that distance is increasingor decreasing with climate change and may be drivenlargely by flow, with larger flow volumes taking longerto reequilibrate with atmospheric conditions (Guet al., 1998). Below the large, lower dam there is aclear shift in the timing of annual maximum streamtemperatures from midsummer toward autumn. Forboth GCMs and all modeled time periods, the lowerdam cools summer stream temperatures, increasesautumn stream temperatures, extends the duration ofwarm temperatures, and increases winter tempera-tures. This study reiterates existing research thatdams alter the thermal regime of rivers in both spaceand time (Olden and Naiman, 2010).

Stream temperature differences between regulatedand unregulated conditions using MIROC32med andNCARPCM1 climate data (Figure 7) further illus-trate the effect of dam regulation. For MIROC32medruns, reservoir releases cool downstream tempera-tures by up to 4°C during June through August;although stream temperatures below reservoirs arenearly 9°C warmer during autumn and early winterwith regulation, illustrating potential changes whencoldwater pools have been depleted. The progressionof time (htp, ntp, mtp, to ftp) shows that changesbecome more pronounced with more severe climatechange. Reservoir releases during summer monthscan cool stream temperatures relatively more withmore extreme climate change (ftp) than for the htp,although climate change causes stream temperaturesto rise overall for both regulated and unregulatedconditions with extreme climate change. In otherwords, in the ftp future, we should expect reservoirreleases to provide a more pronounced cooling effectalthough stream temperature increases. TheNCARPCM1 runs show similar trends, but with lesspronounced summer cooling downstream of reser-voirs. Temperature differences between the

MIROC32med and NCARPCM1 plots representuncertainty from climate modeling assumptions andrepresentation. This modeling indicates changes tostream temperatures from reservoirs of differentsizes and elevations.

Longitudinal Stream Temperature Profiles

Longitudinal stream temperature profiles show thatdams abruptly cool downstream river reaches duringsummer (Figure 8). In headwater reaches in July,streams start with elevated temperatures, particularlyat the northern latitude (Figure 8a, RKM 0 on x-axis).RTEMP struggles to accurately estimate stream tem-perature in headwater reaches with very low flowconditions (<~0.3 m3/s). In real headwater systems,microtopographic shading, hyporheic flow, partialsnow cover at high elevations, and snowmelt likelymoderate stream temperatures somewhat; althoughobserved August stream temperature data show war-mer stream temperatures above high-elevation reser-voirs such as French Meadows and Hell Hole thanbelow them (American watershed) (PCWA, 2007).

Downstream of the middle reservoir, stream temper-atures warm in response to July atmospheric condi-tions. Stream temperatures warm longitudinally belowthe lower reservoir, although at a slower rate thanoccurred below the middle reservoir because flow vol-ume is higher. Overall stream temperatures increasemore with the MIROC32med climate-driven input datathan the NCARPCM1 input data. Regulated streamswarm more quickly than unregulated streams for alllatitudes and time periods because flows are reducedwith regulation — flow in the reach below the middlereservoir is 32-55 m3/s with unregulated conditions,but is 7.5-30 m3/s with regulated conditions. Longitudi-nal heating occurs more rapidly for the regulated runsbecause the river has a slower travel time (i.e., longerexposure to atmospheric conditions) and less mass to beheated.

Figure 8 implies that flow and temperature can bemanaged with dam releases to maintain and enhancesuitable instream habitat for coldwater species withclimate change. Modeling suggests the highest, small-est dam reduced summer stream temperatures theleast because high, small dams generally empty eachyear (do not have carryover storage) and contributerelatively more flow to rivers than lower elevationdams do (typically because river flow is low at upperelevations).

Stream temperatures vary slightly by latitude,although clear trends are not obvious. Differences instream temperatures appear to be sensitive to localdifferences in climate input data rather than differ-ent rates of heating by latitude.

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Limitations

Input Data Uncertainty. Climate and hydrologyinput data were used to estimate instream flow andthe atmospheric heat budget, both of which drivestream temperatures. The cool stream temperaturessuggested by this modeling effort, where maximumaverage annual stream temperatures remain belowapproximately 18°C for all climate alternatives, timeperiods, and management alternatives (regulated vs.unregulated) are representative of these data.

It is beyond the scope of this study to thoroughlyanalyze the climate datasets or to quantify theiraccuracy. However, we have provided a comparison ofmean annual air temperature and precipitation for

both GCMs used in this study as well as the Daymetdataset used in Null et al. (2013) (Table 6). Daymetdata are daily, interpolated surface data (aggregatedto a weekly timestep here) (Thorton et al.,1997). Wealso include projected future change for each modelprojection. Future climate projects from Null et al.(2013) simply model uniform air temperatureincreases of 2, 4, and 6°C as a sensitivity analysis ofunregulated stream temperature response to climatewarming. Table 6 shows that for htp conditions,mean annual air temperature for the central latitudewatershed is nearly 3°C warmer with the Daymetdataset than either GCM, and air temperature iscooler for the NCARPCM1 dataset by the end of the21st Century than Daymet’s htp conditions.

FIGURE 7. Average Weekly Regulated Minus Unregulated Stream Temperature Difference for the Central Watershed for All Time Periods.

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Compounding comparison of input data,MIROC32med and NCARPCM1 climate estimateswere input into the VIC hydrology model (WEAP was

only used to model infrastructure and priorities forwater use with regulated conditions) and Daymet cli-mate data were input into the WEAP hydrologic

FIGURE 8. Average July Stream Temperature Longitudinal Profile for MIROC32med andNCARPCM1 htp and ftp for the (a) Northern, (b) Central, and (C) Southern Latitudes.

TABLE 6. Comparison of Climate Data (future values represent average changebetween each model projection and that model’s simulated historical climate).

ClimateDataset

1970-1990 (historical) 2010-2030 2040-2060 2070-2090

AirTemp, °C

Precip,mm/yr

DAirTemp, °C

DPrecip,%

DAirTemp, °C

DPrecip,%

DAirTemp, °C

DPrecip,%

MIROC32med 8.1 197 1.3 0.8 2.7 �13.3 4.7 �20.1NCARPCM1 8.2 205 0.2 7.5 1.1 6.0 2.1 12.4Daymet 10.9 161 2 0 4 0 6 0

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model (Young et al., 2009). Also, future climateassumptions are different in Figure 9, the GCMsexplicitly model physical processes whereas Daymetdata are increased by 2°C increments as a sensitivityanalysis (all other data and parameters are identical).Thus, both input data and hydrologic modeling weredifferent in this comparison, but it is relevantbecause both methods are common for modeling cli-mate impacts on water resources and to highlight theuncertainty of modeling climate impacts. This com-parison shows that different input data lead to verydifferent stream temperature results (Figure 9). Mod-eled htp stream temperatures with unregulated con-ditions (black lines) differ markedly and highlight thecompounding uncertainty inherent in modeling anti-cipated hydroclimate effects on water quality. Averageannual maximum stream temperature is 23.8°C usingthe Daymet climate data, but is over 10°C cooler, at13.1°C with the MIROC32med climate data. Streamtemperatures reach their average annual maximumin the first week of August for both simulations.Climate is notably difficult to model in mountainregions due to orographic microclimates and thesedata suggest future modeling and research shouldfocus on improving accuracy of climate predictions aswell as calibration efforts for mountain regions.

The input climate and hydrology data discussedabove should be noted as a limitation of this study.More complete input and calibration data wouldimprove results. Calibration of climate and hydrologyinput data has primarily been completed for lowerelevation locations in the Central Valley, with lesscertainty for high-elevation locations of the SierraNevada. While this compromises results, it also high-lights future data needs and is a pertinent finding ofthe study.

Reservoir Operations

This study focuses on stream temperatures notchanges in reservoir operations, and therefore weheld flood control, hydropower, and water supplydemands constant. A limitation of this simplificationis a lack of weekly variability in streamflow thatwould be the result of a more complex hydropowersimulation model. Also, because water and powerdemands do not change with future climate scenarios,our approach does not represent realistic adaptations.Climate change will affect reservoir operations, whichwill further alter reservoir temperatures. Previoushydropower studies in the Sierra Nevada have shownthat reservoirs fill earlier and empty earlier as inflowshifts from springtime snowmelt to winter snowmeltand rain events (Vicuna et al., 2008). The strength ofour method is that it complements existing reservoiroperations studies (that typically ignore water qual-ity) and highlights water temperature changes fromclimate as opposed to reservoir operations. A reser-voir operations model that includes our stream tem-perature modeling module could provide valuableinsights into reservoir operations that include envi-ronmental uses with human uses. Finer temporal res-olution of modeled stream temperatures would alsocomplement hydropower simulation models (whichtypically use hourly or daily timesteps to simulatepeaking), but is not needed for water supply simula-tion models (which often use monthly timesteps).

DISCUSSION AND MAJOR FINDINGS

The thermal regime of rivers has been fundamen-tally altered by humans from water development,land use changes, and increasingly, climate-inducedchanges to atmospheric conditions. Many aquatic eco-systems are thermally sensitive and have had signifi-cant reductions in habitat with these changes. InCalifornia’s Sierra Nevada, coldwater habitat hasgenerally decreased with water development and landuse changes (Yoshiyama et al., 1998). Maintainingcoldwater habitat with anticipated climate change isa future challenge.

Overall, our approach is useful as a proof of con-cept study that can be incorporated with water opera-tions models. The effects of dams on downstreamtemperatures vary by season, stream length betweenreservoirs, reservoir size, outlet structure, and reser-voir elevation. During summer, dams provide a stepreduction in stream temperatures, which then warmlongitudinally in response to atmospheric conditions.Increasingly severe climate change (represented with

FIGURE 9. Modeled Stream Temperatures at the Outlet of theStanislaus River (500 m elevation) for Historical and FutureClimate Conditions Using MIROC32med and Daymet Input Data(T0 is historical conditions and T2, T4, and T6 are 2, 4, and 6°Cclimate warming, respectively) (unregulated stream temperatureswith Daymet input data from Null et al., 2013).

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a longer future outlook) indicates that although theabsolute temperature of stream temperaturesincrease from htp conditions, the thermal reductionfrom reservoir releases grows somewhat more pro-nounced and maintains cool water into summermonths. This implies that reservoirs may be able torelease cool water and offer a potential mitigationstrategy for climate change-induced stream warmingfor short distances downstream. Changing water andpower demands will also impact operations andshould be considered in future work.

In nonstationary climates, the geographic distribu-tions of habitats and ecosystems are also likely tochange (Hanak et al., 2011). This study indicates thatsummertime coldwater habitat is anticipated to existimmediately downstream from dams but stream tem-peratures rise with distance from dams. This indi-cates that coldwater habitat may persist in regulatedrivers with climate change (particularly if dams areoperated to preserve winter water in stratified reser-voirs), although possibly not in the same locations ascoldwater habitat historically existed. This presentsfuture challenges for water managers since many ofthe laws and regulations to protect ecosystems (suchas the Endangered Species Act) were created withthe assumption of ecological stationarity. Given thegenerally cool stream temperatures estimated in thiseffort with the MIROC32med- and NCARPCM-drivenVIC input data (and the incongruence with previ-ously modeled unregulated conditions in west-slopeSierra Nevada rivers) (Null et al., 2013), it is beyondthe scope of this study to analyze the extent of cold-water habitat from river regulation with climatechange.

In California, water is tightly managed. Economic-engineering modeling suggests in a warmer and drierfuture, competition for already scarce water couldincrease by an order of magnitude (Harou et al.,2010). This means that attention should focus onmanaging water quality to maintain and enhanceaquatic ecosystems, especially when maintainingwater quality may reduce the need for environmentalinstream flows (Null et al., 2010b). With increasingwater scarcity, it will be important to understand thefactors limiting aquatic ecosystems in Sierra Nevadarivers and manage those specific problems.

Modeling studies such as this are needed to betterunderstand the thermal effects from dams with cli-mate change and the potential to mitigate for antici-pated changes. Here, we use a novel approach toapply atmospheric projections on a global scale to reg-ulated stream temperatures at the mesoscale. Thisproof of concept study shows the regional modelingapproach is worthwhile for assessing potential forthermal management of reservoir releases to main-tain and enhance downstream coldwater aquatic

ecosystems. Our focus was on relative changes ratherthan absolute stream temperatures to complementexisting reservoir operations research. Incorporatingour approach with reservoir operations studies couldprovide valuable insights into the potential of exist-ing infrastructure to mitigate the effects of climatechange. Future research includes modifying damoutlet works to highlight promising infrastructureimprovements and adaptation strategies for antici-pated hydroclimatic changes. Our method could beapplied to other locations on a case-by-case basisusing input data for the new region, changing reser-voir parameters (such as capacity, outlet structure,depth, elevation, distance between reservoirs…) andrecalibrating models to improve understanding ofregional climate change effects on water quality.

ACKNOWLEDGMENTS

This research was made possible by funding from the CaliforniaEnergy Commission and was submitted as a white paper to themunder the title “Water and Energy Sector Vulnerability to ClimateWarming in the Sierra Nevada: A Method to Consider WhetherDams Mitigate Climate Change Effects on Stream Temperatures”.We would like to thank Jeff Mount and Mike Deas for scholarlycontributions, David Rheinheimer for programming support, aswells as Danielle Salt and Paige Dulberg for downloading mea-sured calibration data, running models, and analyzing data. Wewould also like to acknowledge the Sacramento Municipal UtilityDistrict for sharing measured water temperature data used formodel testing. Thanks to Mary Tyree and the California NevadaApplications Program/California Climate Change Center (directedby the Climate Research Division of Scripps Institute for Oceanog-raphy) for sharing VIC climate data. Finally, we would like tothank Guido Franco and PIER staff for their direction and over-sight, as well as anonymous reviewers for their thoughtful com-ments and suggestions. All authors were affiliated with Universityof California, Davis when the majority of this work took place.

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