CLIMATE IMPACTS ON SURFACE INFLOW AND TILE DRAINAGE IN THE MACKINAW RIVER WATERSHED, CENTRAL ILLINOIS

Understanding the response of water cycle dynamics to climate change and human activity is essential for best management of water resources. This study used the USDA Soil-Water Assessment Tool (SWAT) to measure and predict major water balance variables including stream discharge, potential aquifer recharge, and surface storage in a small-scale watershed (~2,930 km²) in Central Illinois. The watershed is predominantly tile-drained agricultural land, which controls the nutrient dynamics and hydrology. Two reservoirs, Evergreen Lake and Lake Bloomington, and the Mahomet Aquifer in the watershed are used for public water supply. The subject watershed has been very sensitive to recent droughts, such that an interim water supply plan has been developed for water management. To assess how the watershed and tile drainage are affected by future climate change, this study used high-resolution climate projection data (~12 km) in a calibrated and validated SWAT hydrologic model. Using General Circulation Models, four (4) representative concentration pathways (RCPs) developed by the IPCC Coupled Model Intercomparison Project Fifth Assessment Report (CMIP5) were used for prediction of precipitation and temperature for the watershed. Precipitation and temperature are predicted to increase by mid-century for all scenarios. Percentage of precipitation lost as ET decreases for all RCPs, with the exception of RCP 8.5, by 2050. Total aquifer recharge remains consistent temporally. An increase in surface runoff and tile flow is predicted for each RCP by 2050. Total water yield of the watershed increases with each scenario by mid-century. Results indicate continued nutrient loading of the surficial reservoirs that are used for public water supply and recreation. Nutrient management measures will need to remain in place and be enhanced.