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Modeling water temperature dynamics at Shasta Lake, California under the drought conditions of 2015
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Reservoir managers at Shasta Dam in northern California are mandated to provide cold discharge temperatures for endangered Chinook salmon in the downstream Sacramento River. Hydrodynamic modeling of reservoir temperatures has been used to assess reservoir operations at Shasta Reservoir in the past, and can provide insight into reservoir conditions expected in the future. In this study, a CE-QUAL-W2 model of Shasta Reservoir is used to model reservoir temperature conditions under two projected climate change emissions scenarios, and to examine possible reservoir operations that may improve conditions in drought years. The record high air temperature year of 2015 was used as a baseline for simulations. Findings suggest that reservoir water temperatures will be higher under climate change, and that the duration of stratification will increase. Simulated reservoir operations aimed at providing cold discharge temperatures in the drought conditions of 2015 indicate that only extreme reductions in reservoir discharge during springtime resulted in substantial improvements to conditions during the fall of 2015. However, low resolution along the depth profile of the reservoir results in uncertainty in estimates of cold pool volume in the reservoir, a key metric used by reservoir managers in their decision making. High-resolution distributed temperature sensing data collected in 2015/2016 were used to evaluate the ability of increased bathymetric resolution of CE-QUAL-W2 to simulate 2015/2016 temperature conditions in the reservoir. The updated model resolution provided a better estimate of the amount of cold water in the reservoir throughout the year, and can be used in the future to inform decision making at Shasta Dam focused on sustaining Chinook salmon populations.