Evaluating the Potential Effects of Climate Change on Thermal Discharges to a Large-Scale River

Large-scale rivers receive thermal discharges from a variety of sources including electric power plants. This thermal loading dissipates downstream, primarily via radiation, and convective and evaporative heat transfer to the atmosphere. Thermal discharges are typically regulated under the Clean Water Act National Pollutant Discharge Elimination System (NPDES) permit program. The discharge permits usually specify a maximum allowable increase in river temperature due to the discharge, often with the additional criteria of a maximum allowable river temperature. Permit compliance issues are often most critical during summer drought conditions, when the electrical production (and attendant cooling water discharge) is at maximum, ambient river temperatures are high, and river flow is low. Potential climate change could significantly alter the climate and hydrologic variables that are critical to dissipation of thermal loading, and regulation of thermal discharges. Changes in air temperature, humidity, and insolation could directly affect river temperature. Changes in precipitation depth and pattern could alter river low flow characteristics. Further, land-use changes, particularly in agricultural areas, could alter rainfall-runoff response from that which occurs at present. These issues were explored on a large-scale river through use of a hydraulic and energy balance model. The model reach extended for over 200 miles. Historical climate and river flow and temperature data were used to verify model results. Conditions of future climate change were evaluated by downscaling regional climate model projections to locations in the watershed, and using those results to alter the climatic input data to the river of thermal model. Potential changes in river low flow characteristics were evaluated via sensitivity analysis on historical low flows, due to the complexity of land-use/precipitation patterns/runoff rate interaction. The resulting "potential future scenarios" were used to evaluate changes in compliance to typical permit conditions. The analysis indicates that thermal discharge permit compliance could be significantly affected by future climate change conditions. The analysis also suggests additional analyses that would be appropriate to develop a comprehensive strategy for managing thermal loading under climate change conditions.