Regional Groundwater Recharge and Groundwater Evapotranspiration in Illinois

Several recent land surface modeling studies have found that the incorporation of groundwater representation can enhance evapotranspiration due to the additional moisture provided by the aquifer. However, there is a lack of regional-scale field evidence to support this finding. The role of shallow unconfined aquifers in supplying water for the evapotranspiration (groundwater evaporation) is investigated in this study based on a comprehensive 22- year (1984-2005) monthly hydroclimatic dataset in Illinois. State-average monthly groundwater recharge and the separation of streamflow components (surface runoff and groundwater runoff) were estimated by soil water balance computations and compared with the previous estimates. The seasonal and interannual variability of groundwater recharge estimates were investigated. It was found that during the summer, the capillary flux from the shallow water table to the root zone soil moisture helps to maintain a high rate of evapotranspiration (~120 mm/month), and its magnitude increases throughout the summer and reaches the maximum of 12 mm/month in August. Neglecting this mechanism in regional water budget studies or land surface modeling may lead to the underestimation of soil moisture and summer evapotranspiration.