Watershed Modeling to Assessing Impacts of Potential Climate Change on Water Supply Availability

Climate change could have impacts on hydrologic systems threatening, availability of water supply resources. In Illinois, regional water supply planning efforts are attempting to better understand potential impacts on low flow and surface water availability through analysis of hydrologic sensitivity to a range of climate scenarios. This paper explores the development, calibration and validation of Fox River watershed model using the soil and water assessment tool (SWAT) and the model's application to assess impacts of potential climate change. The watershed model is calibrated and validated using daily flow records at three gauging stations. Automatic model calibration followed by manual refinement of parameter values was performed. Calibration results were generally good for monthly and annual time step but only satisfactory for daily simulations. Based on simulations of global climate models produced for IPCC fourth assessment report, climate scenarios were prepared by the Illinois State Water Survey for water supply planning initiatives in north‐east and east‐central Illinois. These scenarios showed ranges of temperature change between 0°C to +3.3°C and annual precipitation changes between -127 to +127 mm in the next 50 years, excluding the 5% extreme ends of those climate model simulations considered. Changes in climate were reflected using adjustments to the historical record, instead of using direct outputs from individual climate models. The watershed model was used to assess the impact of potential climate change. Application results indicate that annual precipitation change of 127 mm on average increases annual water yield and 7‐day low flows by 28% and 19%, respectively. In contrast, a temperature change of +3.3°C results in average reductions of annual water yield by 13% and 7‐day low flows by 10%. Seasonal effects were investigated through evaluation of changes in average monthly flows. Increasing precipitation resulted in significant changes in streamflows in late summer and fall months where as increasing temperature greatly affects winter flows due to snowmelt. The key implication is that climate change‐induced variability of streamflows could have major impacts on water supply availability in the Fox River watershed and in particular, increased periods of drought could result in deficit of supplies during seasons of peak water use. It must be noted that this analysis does not examine the potential impacts of population growth and water use on water supply availability, which are also expected to have substantial influences in the region.