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JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION Vol. 49, No. 6 December 2013 AMERICAN WATER RESOURCES ASSOCIATION 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 of dams with anticipated climate change using a multimodel approach. Coldwater habitat for species such as trout and salmon has been reduced by water regulation, dam building, and land use change that alter stream temperatures. Climate change is an additional threat. Changing hydroclimatic conditions will likely impact water temperatures below dams and affect downstream ecology. We model reservoir thermal dynamics and release operations (assuming that operations remain unchanged through time) of hypothetical reservoirs of different sizes, elevations, and latitudes with climate-forced inflow hydrologies to examine the potential to manage water temperatures for coldwater habitat. All models are one dimensional and operate on a weekly timestep. Results are presented as water temperature change from the historical time period and indicate that reservoirs release water that is cooler than upstream conditions, although the absolute temperatures of reaches below dams warm with 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 stream temperatures and coldwater habitat with climate change. (KEY TERMS: water temperature; water management; climate change; regulated rivers; reservoirs; coldwater habitat; Sierra Nevada.) Null, Sarah E., Scott T. Ligare, and Joshua H. Viers, 2013. A Method to Consider Whether Dams Mitigate Climate Change Effects on Stream Temperatures. Journal of the American Water Resources Association (JAWRA) 49(6): 1456-1472. DOI: 10.1111/jawr.12102 atmosphere and streambed, as well as source temperatures (i.e., groundwater, snowmelt, overland flow, and precipitation). Long-term data analysis has shown that stream temperatures have already increased as air temperatures have risen in recent decades (Hari et al., 2006; Kaushal et al., 2011), and modeling studies suggest that stream temperatures will increase further with climate warming, potentially causing habitat reduction for coldwater species (Jager et al., 1999; Battin et al., 2007; Null et al., 2013). INTRODUCTION AND RATIONALE Stream temperature is an instrumental characteristic of lotic systems, directly influencing dissolved oxygen levels, nutrient cycling, chemical reaction rates, productivity, and mortality in aquatic ecosystems (Poole and Berman, 2001). Stream temperature is influenced by climate, making it particularly vulnerable to climate change (Mohseni and Stefan, 1999). It is driven by heat exchange with the 1 Paper No. JAWRA-12-0146-P of the Journal of the American Water Resources Association (JAWRA). Received June 14, 2012; accepted April 29, 2013. © 2013 American Water Resources Association. Discussions are open until six months from print publication. 2 Respectively, 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. [email protected]). JAWRA 1456 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION A METHOD TO CONSIDER WHETHER DAMS MITIGATE CLIMATE CHANGE EFFECTS OF THE AMERICAN WATER RESOURCES ASSOCIATION STREAM TEMPERATURES mitigating climate change effects on stream temperatures at a regional scale. Reservoirs of varying size, latitude, and elevation in California’s west-slope Sierra Nevada mountain region are used for this proof of concept study. This research begins to quantify the extent to which dams may provide thermal benefit under “normal operations” — a generic operating rule curve that does not change operations to explicitly manage for coldwater. This research is novel because it explores thermal management at the mesoscale, rather than analyze temperatures on a case-by-case basis for individual reservoirs. It is intended to guide operations and help researchers and water managers who are relied on to provide water for multiple and competing uses. This study begins with a description of the Sierra Nevada study area, including a discussion of current water development. We explain our modeling approach, including hydroclimate input data, the Water Evaluation and Planning (WEAP) rainfallrunoff model used to simulate water operations; the Water Quality for River-Reservoir Systems (WQRRS) water quality model used to estimate reservoir temperatures; and the Regional equilibrium stream TEMPerature (RTEMP) model. We discuss reservoir release temperatures with climate change, compare unregulated and regulated stream temperatures, analyze the effect of reservoirs of different sizes, elevations, and latitudes on stream temperature longitudinal profiles, and end with a discussion of data uncertainty. Overall, the approach described here provides a method for dam operators or other stakeholders to incorporate water quality effects of climate change with water supply, hydropower, and flood control impacts for a more complete understanding of climate change impacts of water regulation. Aquatic and riparian habitats are often directly or indirectly managed by water operations and reservoir releases which alter downstream water quantity, quality, and timing. As water is managed more tightly in a future with climate change, this type of research will help to evaluate how reservoir releases may benefit ecosystems. A growing literature links climate change specifically with water quality and quantity for salmonid species (salmon and trout), but does not always consider regulated systems. Research in unregulated systems indicates that stream warming could reduce the total amount 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 to higher elevations or latitudes (Jager et al., 1999; Hari et al., 2006; Isaak et al., 2010; Null et al., 2013), alter the distribution and abundance of coldwater species (Gooseff et al., 2005), or introduce pathways for invasive species (Rahel and Olden, 2008). Most climate change research in regulated systems focuses on impacts to people, such as water supply, hydropower generation, and flood control (Hamlet and Lettenmaier, 1999; Barnett et al., 2005; Medellın-Azuara et al., 2008; Madani and Lund, 2010; Das et al., 2011). It is not well understood the extent to which dams can provide coldwater releases to maintain coldwater habitat in coming decades or how climate change may impact release temperatures. Reservoir releases may present an opportunity to manage water temperature and provide suitable coldwater habitat for fish and wildlife, particularly by releasing water from the coldwater pool of thermally stratified reservoirs. Reservoir releases provide new boundary conditions for downstream reaches, with diminishing influence as distance from the dam increases. It is possible that dams could mitigate some of the stream temperature impacts of climate change, particularly where reservoir releases are colder than unregulated conditions or where winter water is stored in the coldwater pool of deep reservoirs. A handful of articles focus on potential climate warming impacts to stream temperatures 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 climate change effects regulated stream temperatures more if reservoirs release from the surface rather than the deeper coldwater pool. A study of the Blue Mesa Reservoir on the Colorado River showed that climate plays the largest role in reservoir thermal structure and that a warmer climate may reduce the volume of the hypolimnion (Johnson et al., 2004). Yates et al. (2008) modeled California’s Shasta Reservoir and downstream Sacramento River, and determined the dam could provide coldwater to offset the effects of climate change and maintain downstream habitat for native Chinook salmon. Gooseff et al. (2005) found that stream temperatures downstream of a shallow reservoir in Montana’s Lower Madison River are expected to increase from climate change and that they are driven by daytime warming. The objective of this study was to develop a method to evaluate the potential of reservoirs for JOURNAL ON BACKGROUND Study Area California’s west-slope Sierra Nevada rivers flow westward from the crest of the Sierra Nevada through the confluence with the Sacramento-San Joaquin Delta and to the Pacific Ocean (Figure 1). The Sierra Nevada mountain region has a Mediterranean-montane climate with a dry season roughly from May to October 1457 JAWRA NULL, LIGARE, AND VIERS FIGURE 2. Average Annual Number of Weeks Unregulated Stream Temperatures Exceed 24°C (lethal threshold for Chinook salmon and steelhead trout (Eaton and Scheller, 1996)) with Incremental Uniform 2°C Air Temperature Increases (T0, T2, T4, and T6 represent climate warming of 0, 2, 4, and 6°C, respectively) (Null et al., 2013). FIGURE 1. Study Watersheds and Topography (Null et al., 2010a). ture regime, threatening the health and integrity of freshwater ecosystems (Poff, 2009; Olden and Naiman, 2010). West-slope watersheds from the Feather watershed in the north to the Kern watershed in the south have about 24,590 million cubic meters (mcm) of water storage for all dams greater than 1.2 mcm (1 thousand acre feet [taf]), and approximately 8,751 MW 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 approximately equal by elevation, although storage capacity is much greater at lower elevations (Table 1). Roughly 75% of California’s in-state hydropower is produced from more than 150 dams in the Sierra Nevada Mountains at higher elevations (Aspen Environmental and M.Cubed, 2005), which are mostly privately owned and operated solely for hydropower revenues (Vicuna et al., 2008). Winter rain and and a wet season from November to April. Historically, snowline was about 1,000 meters (m). Precipitation averaged about 100 cm/yr for the region, but varied greatly with latitude, elevation, and local weather patterns. Precipitation was greatest in the northern watersheds and the highest elevations of the southern Sierra Nevada. Typically, snowpack persisted late into summer. Modeling suggests that unregulated stream temperatures are anticipated to increase approximately 1.6°C for each 2°C rise in air temperature in this region (simulated with uniform air temperature increases of 2, 4, and 6°C as a sensitivity analysis of stream temperature response to climate warming) (Null et al., 2013) (Figure 2). However, Sierra Nevada rivers have been extensively developed for water supply, hydropower generation, flood protection, and recreation, which has fundamentally altered the natural flow and tempera- TABLE 1. Major Sierra Nevada Reservoirs by Elevation, Latitude, and Average Volume (mcm). Elevation Group Northern (Feather, Yuba, Bear, American, Cosumnes) Very high (>1,600 m) High (800-1,600 m) Mid (<800 m) Total n = 12 avg. vol. n = 14 avg. vol. n=9 avg. vol. n = 35 avg. vol. = 50 = 206 = 800 = 305 Central (Mokelumne, Calaveras, Stanislaus, Tuolumne, Merced) n=4 avg. vol. n=7 avg. vol. n=6 avg. vol. n = 17 avg. vol. Southern (San Joaquin, Kings, Kaweah, Tule, Kern) n=6 avg. vol. n=2 avg. vol. n=7 avg. vol. n = 15 avg. vol. = 82 = 171 = 1,313 = 553 = 137 = 102 = 412 = 261 Total n = 22 avg. vol. n = 23 avg. vol. n = 22 avg. vol. n = 67 avg. vol. = 80 = 186 = 813 = 357 Note: mcm, million cubic meters. JAWRA 1458 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION A METHOD TO CONSIDER WHETHER DAMS MITIGATE CLIMATE CHANGE EFFECTS ON STREAM TEMPERATURES spring snowmelt are stored in relatively small reservoirs that provide within-year storage for generation primarily during summer. The remaining 25% of California’s in-state hydropower is produced in large, low elevation, multiuse projects operated for water supply, hydropower, and flood protection, which are owned and operated by the state and federal governments (Aspen Environmental and M.Cubed, 2005). METHODS Our approach was to model a simple, hypothetical watershed that has many of the physical characteristics found in watersheds of the Sierra Nevada such as hydropower, water supply, and flood control infrastructure, as well as unregulated tributaries. This method required a series of models that, when linked via parameterization and sequencing, can be applied to a large spatial area such as California’s Sierra Nevada and have the ability to represent multiple years and climate scenarios quickly. FIGURE 3. Hypothetical Study Watershed with Modeled Infrastructure. 4,363 mcm (Oroville on the Feather River). We modeled a reservoir approximately the size of Folsom Lake to assess climate change effects on rim dams (Table 2). Flood control operations were developed by simplifying the U.S. Army Corps of Engineers Folsom flood control curve. The top of conservation pool was drawn down to 500 mcm from week 41 to week 46 and held constant for the entire flood season regardless of storage space available upstream. From week 13 to week 21 the flood control space was reduced to zero for the remainder of the year. We represented water supply operations in the lower reservoir with water demand that peaks in the summer and is at a minimum during winter months, based on actual water demand (Department of Water Resources, 2009). We modeled hydropower operations by requiring a minimum release from the reservoir for each week, which was based on average weekly hydropower releases from hydropower plants in the Sierra Nevada (Madani and Lund, 2010). This method assumes that hydropower demand does not change with water year type or climate. To assess the effects of climate change on stream temperatures over a spectrum of latitudes, the entire theoretical watershed was shifted from the central 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 in series. The middle fork is the largest of the three forks and has two hydropower projects, which consist of a reservoir and a diversion to a downstream hydropower plant. Reservoirs are 58.1 mcm (approximately the size of Ice House Reservoir in the American watershed) and 86.9 mcm (about the size of Bowman Lake in the Yuba watershed), respectively (Table 2). The north fork and the south fork are undammed and have a natural flow regime. The lowest reservoir is below the confluence of all three forks near the bottom of the watershed representing a multipurpose “rim” dam, managed for water supply, hydropower, and flood control. The size of rim reservoirs in the Sierra Nevada varies from 86.3 mcm (Englebright on the Yuba River) to over TABLE 2. Physical Characteristics of Modeled Reservoirs. Reservoir Elevation Upper Middle Lower Reservoir Size Elevation (m) Storage Capacity (mcm) Hydropower Capacity (MW) Turbine Capacity (m3/s) S M L 1,615 1,291 300 58.1 86.9 1,233.0 12 65 200 9.0 25.5 86.8 Note: mcm, million cubic meters. JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 1459 JAWRA NULL, LIGARE, AND WEAP, WQRRS, and RTEMP were run sequentially, and are described below (see modeling sequence in right side of Figure 4). A weekly timestep was used for all models. This study required 48 model runs, representing two GCMs, two management scenarios for regulated and unregulated conditions, three latitudes, and four 20-year time periods ranging from 1970 to 2090, which were labeled as historical (htp), near-term (ntp), mid-term (mtp), and far-term (ftp) time periods (Table 4). Sierra Nevada (in the Stanislaus River) about 200 km north to the approximate location of the Feather watershed and about 100 km south to the approximate location of the San Joaquin watershed (Figure 4). Watershed size, shape, and infrastructure remained constant and only climatic inputs and hydrology were changed with latitude. Input Data and Modeling Approach Reservoir Operations and Outflows — Water Evaluation and Planning. WEAP is a water management and hydrology software package developed by the Stockholm Environment Institute. Here, hydrology was modeled with the VIC model by the California Nevada Applications Program/California Climate Change Center and WEAP was used to model water management including infrastructure, water demands, and allocation priorities (Yates et al., 2005). It operates on a weekly timestep with spatial representation of 250-m elevation bands (Young et al., 2009). We placed the highest allocation priority on water supply, second on hydropower generation, and third on reservoir storage. WEAP output data were passed to RTEMP and WQRRS (Figure 4). See Yates et al. (2005) for WEAP governing equations and additional model detail, Young et al. (2009) for a description of unregulated Sierra Nevada flow modeling, Null et al. (2010a) for hydrologic sensitivity to climate warming in California’s Sierra Nevada, and Mehta et al. (2011) for climate change impacts on Sierra Nevada hydropower production. We used two general circulation models (GCMs) that represent extremes for possible precipitation and temperature, MIROC32med and NCARPCM1, MIROC32med is warmer and drier whereas NCARPCM1 is cooler and wetter (Cayan et al., 2009). Both GCMs used the A2 emissions scenario, which assumes moderately severe climate change and continuously increasing population growth (Weare, 2009). GCM output was downscaled to 1/8° using the bias correction and spatial downscaling method described in Maurer et al. (2010). Climate input data drove the Variable Infiltration Capacity (VIC) rainfall-runoff model to produce surface runoff, base flow, evapotranspiration, and soil moisture on a 1/8° spatially gridded format (Maurer, 2007). These data were provided by the California Nevada Applications Program/California Climate Change Center (Table 3). Reservoir Water Quality Modeling — Water Quality for Reservoir-River Systems. Reservoir temperatures were simulated with WQRRS, a one-dimensional river and reservoir model developed by the U.S. Army Corps of Engineers (USACE-HEC, 1986). WQRRS is a physically-based, finite difference model based on the principles of conservation of heat and mass. The model operates on a daily timestep, although we used aggregated weekly input and output data (we input average weekly inflow, stream temperature, and weather data and used average weekly water temperature output). We chose WQRRS because of quick run times (20 years were simulated in seconds). For this study, we used only the reservoir simulation component (not the river simulation component so that unregulated stream temperatures previously modeled with RTEMP were comparable). Water temperature was the only water quality parameter modeled and was estimated using the heat budget method (USACE-HEC, 1986). WQRRS has been used extensively through the past three decades to model water quality in lakes FIGURE 4. West-Slope Watersheds of the Sierra Nevada Showing Locations of Hypothetical Watersheds and Reservoirs (left side) and Model Run Flow chart (right side). JAWRA VIERS 1460 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION A METHOD TO CONSIDER WHETHER DAMS MITIGATE CLIMATE CHANGE EFFECTS ON STREAM TEMPERATURES TABLE 3. Model Input Data and Source. RTEMP Input from Null et al. (2013), WEAP Input from Young et al. (2009). Model Input VIC Source GCM (MIROC32med and NCARPCM1–A2 SRES) boundary conditions Meteorology Hydrology (surface runoff, snow accumulation, snowmelt, groundwater) Topography Reservoir outflow Reservoir release water temperature Inflow hydrology Reservoir operational rules Meteorology RTEMP WEAP WQRRS Geometry Flow (inflow, outflow, initial storage) Inflow water temperature California Climate Change Center VIC (cloud cover user defined) VIC USGS 10 m digital elevation model WEAP WQRRS VIC User defined VIC (wet bulb temperature calculated, cloud cover user defined) User defined WEAP 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 Climate models Time periods Latitudes Management1 Model Runs Count A2-MIROC32med A2-NCARPCM1 htp (1970-1990) ntp (2010-2030) mtp (2040-2060) ftp (2070-2090) Northern Central Southern Regulated Unregulated 2 Total model runs V 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 sources and sinks (kcal/s). Internal transport of heat and mass occurs only in the vertical direction through advection and effective diffusion mechanisms, with water well mixed laterally and longitudinally. We assumed all reservoir inflow occurs at the upstream end of the reservoir, making the rate of advection slower than if the inflow occurred closer to the outlet. Effective diffusion between layers is based on temperature and includes molecular and turbulent diffusion as well as convective mixing based on density gradients. One-dimensional models such as WQRRS work well for small to large reservoirs, but should not be used for shallow reservoirs or reservoirs with small residence times (i.e., vertically mixed systems that would be better represented with a river model) (USACE-HEC, 1986). Reservoirs are represented as a series of onedimensional layers, with user-defined surface area, thickness, and volume. For this application, layer thickness was 1 m. Inflows are instantaneously dispersed throughout the reservoir layer of similar density. Outflows are modeled using the selective withdrawal allocation method developed by the U.S. Army Engineer Waterways Experiment Station (WES method) (USACE-HEC, 1986) to describe the vertical limits of the withdrawal zone and the vertical velocity distribution within the zone. For additional model 4 3 2 48 Notes: htp, historical period; ntp, near-term period; mtp, mid-term period; ftp, far-term period. 1 Reservoir operation identical for all regulated runs (generic operating rules). and reservoirs, with recent publications reviewed here. It was used to aid decision making by evaluating management alternatives for sustainable ecosystem management of Turkish coastal lagoons (Gonenc et al., 1997), and to aid conflict resolution over water allocation by modeling Iran’s Karkheh river-reservoir system (Karamouz et al., 2008). Deas and Orlob (1999) simulated water quantity and quality for restoration of anadromous fisheries below Iron Gate Dam on the Klamath River. Lopes et al. (2003) used it to simulate dissolved oxygen and water temperature in Portugal’s Aguieira Reservoir, and Meyer et al. (1996) altered meteorologic and hydrologic input conditions to estimate water quality effects on Shasta Reservoir and part of the Sacramento River from a doubling of CO2. Water temperature and other water quality parameters are simulated using the one-dimensional form of the advection-dispersion equation: JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION @C @C @2C þ DxQx ¼ DxAx Dc 2 þ Qi Ci Qo C VS ð1Þ @t @x @x 1461 JAWRA NULL, LIGARE, AND detail, see USACE-HEC (1986). Here, we aimed to represent reservoir outlet structures in a generic way, and modeled withdrawal outlets as a small (<1 m3/s) withdrawal structure near the bed of the reservoir and a second outlet near the top of the coldwater pool for the remainder of outflow (modeled with WEAP). Spill events occur from the reservoir surface. In reality, there is quite a bit of variability in outlet structure design. The quantity and quality of reservoir releases form the boundary conditions for downstream river temperature models. See Table 3 for a list of input data and sources used for each reservoir model. Results from WEAP and WQRRS are passed to RTEMP to estimate stream temperatures, and the models are run sequentially (Figure 4). from WEAP and WQRRS, we assumed that snowmelt was 1°C to account for slight heating during overland flow from snowpatch to stream, precipitation temperature was the same as air temperature, and groundwater temperature was 7.5°C, which is the mean annual air temperature for the west-slope Sierra Nevada at 2,500 m. Equilibrium temperature is useful because heat flux at the water surface can be represented with only meteorological input variables, and it can be applied on a regional scale. RTEMP has previously been used to assess unregulated stream temperature sensitivity to climate warming in California’s westslope Sierra Nevada, with an emphasis on impacts to coldwater habitat for anadromous salmon species (Null et al., 2013). Other equilibrium temperature studies exist. Notable examples include assessing the effects of climate change by examining the relationship between stream and air temperatures (Mohseni and Stefan, 1999), predicting stream temperatures in Canada’s Miramichi River (Caissie et al., 2005), modeling Colorado River temperatures downstream of Glen Canyon Dam (Wright et al., 2009), and quantifying the effects of stream discharge on summer river temperature (Gu et al., 1998). Stream Temperature Model — Regional Equilibrium Temperature Model. Stream temperatures were estimated with RTEMP, a one-dimensional equilibrium temperature model developed by Watercourse Engineering, Inc. (Null et al., 2013). We used a weekly timestep and spatial resolution of 250-m vertical elevation bands for streams (the same resolution as WEAP). RTEMP is a physically-based model, which uses meteorology, hydrology, boundary conditions, and stream length (Table 3) to estimate stream temperatures using equilibrium temperature theory (Edinger et al., 1968). Stream temperatures are estimated using a simplified form of the advection-dispersion equation: @Tw Hnet ¼ @t Cp qd Model Calibration and Testing WEAP and RTEMP models were previously calibrated for the 15 west-slope Sierra Nevada watersheds with natural flow hydrologic conditions (Young et al., 2009; Null et al., 2013). Reservoir water temperatures, modeled with WQRRS, were calibrated using measured temperature profiles from Icehouse Reservoir, Bowman Reservoir, and Folsom Reservoir for the upper, middle, and lower reservoirs, respectively. Calibration dates differ between modeled reservoirs depending on when measured data were available. Most stream and reservoir temperature studies in the Sierra Nevada are part of Federal Energy Regulatory Commission (FERC) relicensing, and therefore are limited to short-term collection efforts. The upper reservoir calibration was completed using 13 thermal profiles. Daily average stream inflow and outflow temperatures from October 1, 2000 to October 19, 2004, were collected by Sacramento Municipal Utilities District (SMUD, unpublished data) and observed streamflow measurements were from the U.S. Geological Survey (USGS) (USGS, 2012). The middle reservoir was calibrated with two years of measured data from Nevada Irrigation District’s (NID) Yuba-Bear project (NID, unpublished data). Inflow and outflow temperature data included 15-min temperature observations from June 3, 2008 ð2Þ where Tw is water temperature (°C), t is time (wk), Hnet is net heat exchange at both the bed- and airwater interfaces (W/m2), Cp is the specific heat of water (4,185 J/kg/°C), q is water density (1,000 kg/m3), and d is water depth (m). Equation (2) assumes that rivers are advection dominated and that changes in longitudinal stream temperatures are driven by meteorological conditions, which alter stream temperatures with respect to time. It further assumes a rectangular channel where water surface is the same width as the streambed. For a full derivation and additional model detail, see Null et al. (2013). Equation (2) produces a time series of dynamic stream temperatures where water seeks equilibrium temperature, but is constrained by boundary conditions, travel time, and river geometry. Stream temperatures are estimated separately for tributaries and mainstem rivers. This allows attenuation of tributaries to be assessed. Mainstem and tributary channels mix at the bottom of each reach using a mass balance approach. In addition to boundary conditions for flow and stream temperature obtained JAWRA VIERS 1462 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION A METHOD TO CONSIDER WHETHER DAMS MITIGATE CLIMATE CHANGE EFFECTS ON STREAM TEMPERATURES flow hydrologic conditions using monthly natural flow estimates at watershed outlets, and snow water equivalent measurements at upstream sites within each watershed (Young et al., 2009). Overall, WEAP represents flow hydrographs well at the watershed scale, and mean bias between simulated and measured natural flow at watershed outlets was 1%, with a range 6 to 2%. RTEMP models were also previously calibrated for unregulated conditions and are described in detail in Null et al. (2013). Simulated stream temperatures fit measured data well and the timing of seasonal maximum temperatures was consistent between modeled and observed data. MAE was within 1.2°C of measured maximum or minimum temperatures, with a mean of 0.4°C. to October 10, 2008, and June 12, 2009 to October 16, 2009. Reservoir thermal profile measurements were collected by NID in July, August, September, and October of 2008 and 2009. Inflow discharge was estimated with USGS measured daily outflows through the Bowman-Spaulding Canal and Canyon Creek below Spaulding (USGS, 2012). The lower reservoir was calibrated with Folsom Lake inflow and outflow, as well as stream temperature data from the American River below Folsom Reservoir from September 30, 2000 to April 10, 2011 (CDEC, 2011). No thermal profiles for Folsom Reservoir were available for calibration. Evaporation coefficients and radiation absorption coefficients were adjusted so modeled profiles and outlet temperatures reasonably matched observed data, and represent observed stratification and mixing within each reservoir. Regulated Stream Temperatures in a Changing Climate Study findings focus on reservoir release temperatures with climate change for reservoirs of different sizes and elevations. Results also compare regulated stream temperatures with unregulated stream temperatures to highlight how climate change may affect regulated systems differently than unregulated systems. We discuss study results by focusing on the central latitude, although the northern, central, and southern latitudes are compared with longitudinal stream temperature profiles at the end of this section. Major findings from this work illustrate the utility of our approach as a proof of concept study. Overall, modeled stream temperatures with anticipated climate change representing ftp conditions are cooler than current observed stream temperatures. We discuss this and provide a brief analysis of input data at the end of this section. RESULTS Overall, vertical temperature profiles within reservoirs matched observations well, indicating that models capture the timing and depth of thermal stratification (see Null et al., 2012 for figures of modeled and observed vertical temperature profiles). Mean bias, mean absolute error (MAE), and root mean square error (RMSE) were calculated for reservoir release temperatures to evaluate WQRRS performance for the upper, middle (measured data from two similar sites were compared), and lower reservoirs. MAE was always less than 2.4°C and averaged 1.8°C for all sites with measured data. Calibration statistics are summarized in Table 5. Reservoir release temperatures are important for stream temperatures because they provide the boundary conditions for downstream temperatures and thus can drive thermal response. WEAP models were previously calibrated for the 15 west-slope Sierra Nevada watersheds with natural Reservoir Release Temperatures. Modeling suggests that average reservoir release temperatures will be resilient to climate change through winter (Figure 5). Reservoir release temperatures warm TABLE 5. Calibration Model Fit Statistics by Reservoir. Upper Measured Location n, count Mean bias, °C MAE, °C RMSE, °C Calibration date, month/date/year Middle Lower Icehouse Release Bowman-Spaulding Canal Canyon Creek below Spaulding Folsom Release 187 0.1 1.1 1.5 10/1/00-10/19/04 40 0.7 1.9 2.3 6/3/08-10/10/08, 6/12/09-10/16/09 66 0.9 2.4 3.0 6/3/08-10/10/08, 6/12/09-10/16/09 547 0.7 1.6 2.0 9/30/00-4/10/11 Note: MAE, mean absolute error; RMSE, root mean square error. JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 1463 JAWRA NULL, LIGARE, AND VIERS FIGURE 5. Average Weekly Central Latitude MIROC32med and NCARPCM1 Reservoir Release Temperatures by Time Period for the (a) Upper, (b) Middle, and (c) Lower Reservoirs combination of outlet works infrastructure, reservoir operations, depth, and inflow temperatures. Because the middle reservoir is calibrated to Bowman Reservoir, some of the above factors are implicitly included in this analysis. In addition, the modeled middle reservoir (Figure 5b) is deeper than the upper reservoir (Figure 5a), potentially affecting the volume of the cold-water pool, with cooler release temperatures during summer. The middle reservoir illustrates the importance of parameterizing and calibrating models for specific systems or regions when using this approach. On average, the warmest water is released in August to early September for the upper reservoir, late July to early August for the middle reservoir, and early October for the large, lower reservoir (Figure 5). However, there is considerable variability between years that is not depicted in the annually averaged figures. For the upper reservoir, the timing of the warmest releases generally became more consistent with more severe climate change (mtp and from climate change beginning in early May. Springtime warming was particularly evident at the upper and middle reservoirs (Figures 5a and 5b), indicating that the effects of climate change on reservoir releases may be more severe at higher elevations. Simulations using MIROC32med input climate data were warmer than those using NCARPCM1 data (MIROC32med predicts a warmer and drier future than NCARPCM1). In fact, release temperatures for NCARPCM1 ftp were similar to release temperatures for MIROC32med ntp (Figure 5). This is indicative of uncertainty in climate modeling, such as representation of physical processes and sensitivity to greenhouse gas forcings. Reservoir release temperatures were cooler for the middle reservoir than the upper reservoir, reflecting model calibration and reservoir depth. The upper and middle reservoirs were calibrated to Ice House and Bowman Reservoirs, respectively. Bowman Reservoir had exceptionally cold release temperatures (much colder than Ice House Reservoir), a result of the JAWRA 1464 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION A METHOD TO CONSIDER WHETHER DAMS MITIGATE CLIMATE CHANGE EFFECTS ON STREAM TEMPERATURES outlook into the future (Figure 6, first and third columns, time progresses from the htp, ntp, mtp, and ftp in the downward direction). For all unregulated runs, the middle river that reaches from approximately 150-250 river kilometer (RKM) is the warmest, before tributaries subsequently join the mainstem with cooler temperatures and higher flows that moderate mainstem temperatures. Toward the end of the 21st Century, modeling indicates that streams not only become warmer, but that the average duration with warmer temperatures is extended as well (x-axes in Figure 6). Unregulated winter stream temperatures become approximately 2°C warmer as well. For MIROC32med unregulated simulations, maximum average weekly ftp). For all reservoirs, the warmest release temperatures coincided with extended periods of low reservoir storage which forced mixing of stratified waters within the reservoir, or less commonly, with spill events. The upper reservoir typically had less stored water earlier in the year than the large, lower reservoir. For the lower reservoir, low reservoir levels most commonly occurred in early fall, which is consistent with observed storage levels in large rim dams (CDEC, 2011). Effects of Reservoirs on Stream Temperatures. For unregulated conditions, modeling suggests that stream temperatures warm with more severe climate change, represented as a farther FIGURE 6. Modeled Average Weekly Stream Temperatures for the Central Latitude with Regulated and Unregulated Conditions for Both GCMs and All Time Periods. JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 1465 JAWRA NULL, LIGARE, AND MIROC32med and NCARPCM1 plots represent uncertainty from climate modeling assumptions and representation. This modeling indicates changes to stream temperatures from reservoirs of different sizes and elevations. stream temperatures are 13.1, 14.4, 15.8, and 18.2°C for htp, ntp, mtp, and ftp, respectively. For unregulated NCARPCM1 runs, stream temperatures are 13.2, 13.2, 14.3, and 14.8°C for htp, ntp, mtp, and ftp, respectively. The warmer stream temperatures in the MIROC32med runs are largely driven by air temperatures that increase at double the rate of NCARPCM1 air temperatures for the ntp, mtp, and ftp. The relatively cool reservoir releases of the upper and middle reservoirs (Figure 5) provide cooler stream temperatures during summer and/or a shorter duration of warm stream temperatures than occur with unregulated conditions. However, the longitudinal length of cooler stream temperatures below dams is variable (Figure 6 — second and fourth columns). For example, cooler temperatures below the middle reservoir vary by approximately 10-90 RKM depending on GCM input data and modeled time period. There is no clear trend as to whether that distance is increasing or decreasing with climate change and may be driven largely by flow, with larger flow volumes taking longer to reequilibrate with atmospheric conditions (Gu et al., 1998). Below the large, lower dam there is a clear shift in the timing of annual maximum stream temperatures from midsummer toward autumn. For both GCMs and all modeled time periods, the lower dam cools summer stream temperatures, increases autumn stream temperatures, extends the duration of warm temperatures, and increases winter temperatures. This study reiterates existing research that dams alter the thermal regime of rivers in both space and time (Olden and Naiman, 2010). Stream temperature differences between regulated and unregulated conditions using MIROC32med and NCARPCM1 climate data (Figure 7) further illustrate the effect of dam regulation. For MIROC32med runs, reservoir releases cool downstream temperatures by up to 4°C during June through August; although stream temperatures below reservoirs are nearly 9°C warmer during autumn and early winter with regulation, illustrating potential changes when coldwater pools have been depleted. The progression of time (htp, ntp, mtp, to ftp) shows that changes become more pronounced with more severe climate change. Reservoir releases during summer months can cool stream temperatures relatively more with more extreme climate change (ftp) than for the htp, although climate change causes stream temperatures to rise overall for both regulated and unregulated conditions with extreme climate change. In other words, in the ftp future, we should expect reservoir releases to provide a more pronounced cooling effect although stream temperature increases. The NCARPCM1 runs show similar trends, but with less pronounced summer cooling downstream of reservoirs. Temperature differences between the JAWRA VIERS Longitudinal Stream Temperature Profiles Longitudinal stream temperature profiles show that dams abruptly cool downstream river reaches during summer (Figure 8). In headwater reaches in July, streams start with elevated temperatures, particularly at the northern latitude (Figure 8a, RKM 0 on x-axis). RTEMP struggles to accurately estimate stream temperature in headwater reaches with very low flow conditions (<~0.3 m3/s). In real headwater systems, microtopographic shading, hyporheic flow, partial snow cover at high elevations, and snowmelt likely moderate stream temperatures somewhat; although observed August stream temperature data show warmer stream temperatures above high-elevation reservoirs such as French Meadows and Hell Hole than below them (American watershed) (PCWA, 2007). Downstream of the middle reservoir, stream temperatures warm in response to July atmospheric conditions. Stream temperatures warm longitudinally below the lower reservoir, although at a slower rate than occurred below the middle reservoir because flow volume is higher. Overall stream temperatures increase more with the MIROC32med climate-driven input data than the NCARPCM1 input data. Regulated streams warm more quickly than unregulated streams for all latitudes and time periods because flows are reduced with regulation — flow in the reach below the middle reservoir is 32-55 m3/s with unregulated conditions, but is 7.5-30 m3/s with regulated conditions. Longitudinal heating occurs more rapidly for the regulated runs because the river has a slower travel time (i.e., longer exposure to atmospheric conditions) and less mass to be heated. Figure 8 implies that flow and temperature can be managed with dam releases to maintain and enhance suitable instream habitat for coldwater species with climate change. Modeling suggests the highest, smallest dam reduced summer stream temperatures the least because high, small dams generally empty each year (do not have carryover storage) and contribute relatively more flow to rivers than lower elevation dams do (typically because river flow is low at upper elevations). Stream temperatures vary slightly by latitude, although clear trends are not obvious. Differences in stream temperatures appear to be sensitive to local differences in climate input data rather than different rates of heating by latitude. 1466 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION A METHOD TO CONSIDER WHETHER DAMS MITIGATE CLIMATE CHANGE EFFECTS ON STREAM TEMPERATURES FIGURE 7. Average Weekly Regulated Minus Unregulated Stream Temperature Difference for the Central Watershed for All Time Periods. both GCMs used in this study as well as the Daymet dataset used in Null et al. (2013) (Table 6). Daymet data are daily, interpolated surface data (aggregated to a weekly timestep here) (Thorton et al.,1997). We also include projected future change for each model projection. Future climate projects from Null et al. (2013) simply model uniform air temperature increases of 2, 4, and 6°C as a sensitivity analysis of unregulated stream temperature response to climate warming. Table 6 shows that for htp conditions, mean annual air temperature for the central latitude watershed is nearly 3°C warmer with the Daymet dataset than either GCM, and air temperature is cooler for the NCARPCM1 dataset by the end of the 21st Century than Daymet’s htp conditions. Limitations Input Data Uncertainty. Climate and hydrology input data were used to estimate instream flow and the atmospheric heat budget, both of which drive stream temperatures. The cool stream temperatures suggested by this modeling effort, where maximum average annual stream temperatures remain below approximately 18°C for all climate alternatives, time periods, and management alternatives (regulated vs. unregulated) are representative of these data. It is beyond the scope of this study to thoroughly analyze the climate datasets or to quantify their accuracy. However, we have provided a comparison of mean annual air temperature and precipitation for JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 1467 JAWRA NULL, LIGARE, AND VIERS FIGURE 8. Average July Stream Temperature Longitudinal Profile for MIROC32med and NCARPCM1 htp and ftp for the (a) Northern, (b) Central, and (C) Southern Latitudes. TABLE 6. Comparison of Climate Data (future values represent average change between each model projection and that model’s simulated historical climate). 1970-1990 (historical) Climate Dataset MIROC32med NCARPCM1 Daymet 2010-2030 2040-2060 Air Temp, °C Precip, mm/yr DAir Temp, °C DPrecip, % DAir Temp, °C DPrecip, % DAir Temp, °C DPrecip, % 8.1 8.2 10.9 197 205 161 1.3 0.2 2 0.8 7.5 0 2.7 1.1 4 13.3 6.0 0 4.7 2.1 6 20.1 12.4 0 Compounding comparison of input data, MIROC32med and NCARPCM1 climate estimates were input into the VIC hydrology model (WEAP was JAWRA 2070-2090 only used to model infrastructure and priorities for water use with regulated conditions) and Daymet climate data were input into the WEAP hydrologic 1468 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION A METHOD TO CONSIDER WHETHER DAMS MITIGATE CLIMATE CHANGE EFFECTS This study focuses on stream temperatures not changes in reservoir operations, and therefore we held flood control, hydropower, and water supply demands constant. A limitation of this simplification is a lack of weekly variability in streamflow that would be the result of a more complex hydropower simulation model. Also, because water and power demands do not change with future climate scenarios, our approach does not represent realistic adaptations. Climate change will affect reservoir operations, which will further alter reservoir temperatures. Previous hydropower studies in the Sierra Nevada have shown that reservoirs fill earlier and empty earlier as inflow shifts from springtime snowmelt to winter snowmelt and rain events (Vicuna et al., 2008). The strength of our method is that it complements existing reservoir operations studies (that typically ignore water quality) and highlights water temperature changes from climate as opposed to reservoir operations. A reservoir operations model that includes our stream temperature modeling module could provide valuable insights into reservoir operations that include environmental uses with human uses. Finer temporal resolution of modeled stream temperatures would also complement hydropower simulation models (which typically use hourly or daily timesteps to simulate peaking), but is not needed for water supply simulation models (which often use monthly timesteps). DISCUSSION AND MAJOR FINDINGS The thermal regime of rivers has been fundamentally altered by humans from water development, land use changes, and increasingly, climate-induced changes to atmospheric conditions. Many aquatic ecosystems are thermally sensitive and have had significant reductions in habitat with these changes. In California’s Sierra Nevada, coldwater habitat has generally decreased with water development and land use changes (Yoshiyama et al., 1998). Maintaining coldwater habitat with anticipated climate change is a future challenge. Overall, our approach is useful as a proof of concept study that can be incorporated with water operations models. The effects of dams on downstream temperatures vary by season, stream length between reservoirs, reservoir size, outlet structure, and reservoir elevation. During summer, dams provide a step reduction in stream temperatures, which then warm longitudinally in response to atmospheric conditions. Increasingly severe climate change (represented with FIGURE 9. Modeled Stream Temperatures at the Outlet of the Stanislaus River (500 m elevation) for Historical and Future Climate Conditions Using MIROC32med and Daymet Input Data (T0 is historical conditions and T2, T4, and T6 are 2, 4, and 6°C climate warming, respectively) (unregulated stream temperatures with Daymet input data from Null et al., 2013). OF THE AMERICAN WATER RESOURCES ASSOCIATION STREAM TEMPERATURES Reservoir Operations model (Young et al., 2009). Also, future climate assumptions are different in Figure 9, the GCMs explicitly model physical processes whereas Daymet data are increased by 2°C increments as a sensitivity analysis (all other data and parameters are identical). Thus, both input data and hydrologic modeling were different in this comparison, but it is relevant because both methods are common for modeling climate impacts on water resources and to highlight the uncertainty of modeling climate impacts. This comparison shows that different input data lead to very different stream temperature results (Figure 9). Modeled htp stream temperatures with unregulated conditions (black lines) differ markedly and highlight the compounding uncertainty inherent in modeling anticipated hydroclimate effects on water quality. Average annual maximum stream temperature is 23.8°C using the Daymet climate data, but is over 10°C cooler, at 13.1°C with the MIROC32med climate data. Stream temperatures reach their average annual maximum in the first week of August for both simulations. Climate is notably difficult to model in mountain regions due to orographic microclimates and these data suggest future modeling and research should focus on improving accuracy of climate predictions as well as calibration efforts for mountain regions. The input climate and hydrology data discussed above should be noted as a limitation of this study. More complete input and calibration data would improve results. Calibration of climate and hydrology input data has primarily been completed for lower elevation locations in the Central Valley, with less certainty for high-elevation locations of the Sierra Nevada. While this compromises results, it also highlights future data needs and is a pertinent finding of the study. JOURNAL ON 1469 JAWRA NULL, LIGARE, AND ecosystems. Our focus was on relative changes rather than absolute stream temperatures to complement existing reservoir operations research. Incorporating our approach with reservoir operations studies could provide valuable insights into the potential of existing infrastructure to mitigate the effects of climate change. Future research includes modifying dam outlet works to highlight promising infrastructure improvements and adaptation strategies for anticipated hydroclimatic changes. Our method could be applied to other locations on a case-by-case basis using input data for the new region, changing reservoir parameters (such as capacity, outlet structure, depth, elevation, distance between reservoirs…) and recalibrating models to improve understanding of regional climate change effects on water quality. a longer future outlook) indicates that although the absolute temperature of stream temperatures increase from htp conditions, the thermal reduction from reservoir releases grows somewhat more pronounced and maintains cool water into summer months. This implies that reservoirs may be able to release cool water and offer a potential mitigation strategy for climate change-induced stream warming for short distances downstream. Changing water and power demands will also impact operations and should be considered in future work. In nonstationary climates, the geographic distributions of habitats and ecosystems are also likely to change (Hanak et al., 2011). This study indicates that summertime coldwater habitat is anticipated to exist immediately downstream from dams but stream temperatures rise with distance from dams. This indicates that coldwater habitat may persist in regulated rivers with climate change (particularly if dams are operated to preserve winter water in stratified reservoirs), although possibly not in the same locations as coldwater habitat historically existed. This presents future challenges for water managers since many of the laws and regulations to protect ecosystems (such as the Endangered Species Act) were created with the assumption of ecological stationarity. Given the generally cool stream temperatures estimated in this effort with the MIROC32med- and NCARPCM-driven VIC input data (and the incongruence with previously modeled unregulated conditions in west-slope Sierra Nevada rivers) (Null et al., 2013), it is beyond the scope of this study to analyze the extent of coldwater habitat from river regulation with climate change. In California, water is tightly managed. Economicengineering modeling suggests in a warmer and drier future, competition for already scarce water could increase by an order of magnitude (Harou et al., 2010). This means that attention should focus on managing water quality to maintain and enhance aquatic ecosystems, especially when maintaining water quality may reduce the need for environmental instream flows (Null et al., 2010b). With increasing water scarcity, it will be important to understand the factors limiting aquatic ecosystems in Sierra Nevada rivers and manage those specific problems. Modeling studies such as this are needed to better understand the thermal effects from dams with climate change and the potential to mitigate for anticipated changes. Here, we use a novel approach to apply atmospheric projections on a global scale to regulated stream temperatures at the mesoscale. This proof of concept study shows the regional modeling approach is worthwhile for assessing potential for thermal management of reservoir releases to maintain and enhance downstream coldwater aquatic JAWRA VIERS ACKNOWLEDGMENTS This research was made possible by funding from the California Energy Commission and was submitted as a white paper to them under the title “Water and Energy Sector Vulnerability to Climate Warming in the Sierra Nevada: A Method to Consider Whether Dams Mitigate Climate Change Effects on Stream Temperatures”. We would like to thank Jeff Mount and Mike Deas for scholarly contributions, David Rheinheimer for programming support, as wells as Danielle Salt and Paige Dulberg for downloading measured calibration data, running models, and analyzing data. We would also like to acknowledge the Sacramento Municipal Utility District for sharing measured water temperature data used for model testing. Thanks to Mary Tyree and the California Nevada Applications Program/California Climate Change Center (directed by the Climate Research Division of Scripps Institute for Oceanography) for sharing VIC climate data. Finally, we would like to thank Guido Franco and PIER staff for their direction and oversight, as well as anonymous reviewers for their thoughtful comments and suggestions. All authors were affiliated with University of California, Davis when the majority of this work took place. LITERATURE CITED Aspen Environmental and M.Cubed, 2005. 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