Regional Groundwater Evapotranspiration in Illinois

The role of shallow unconfined aquifers in supplying water for evapotranspiration (i.e., groundwater evaporation) is investigated in this paper. Recent results from regional land surface modeling have indicated that in shallow water table areas, a large portion of evapotranspiration comes directly from aquifers. However, little field evidence at the regional scale has been reported to support this finding. Using a comprehensive 19-yr (1984-2002) monthly hydrological dataset on soil moisture, water table depth, and streamflow in Illinois, regional recharge to and evaporation from groundwater are estimated by using soil water balance computation. The 19-yr mean groundwater recharge is estimated to be 244 mm yr(-1) (25% of precipitation), with uncertainty ranging from 202 to 278 mm yr(-1). During the summer, the upward capillary flux from the shallow aquifer helps to maintain a high rate of evapotranspiration. Groundwater evaporation (negative groundwater recharge) occurs during the period of July-September, with a total of 31.4 mm (10% of evapotranspiration). Analysis of the relative soil saturation at 11 depths from 0 to 2 m deep supports the dominance of groundwater evaporation across the water table in dry periods. The zero-flux plane separating the recharge zone from the evapotranspiration zone propagates downward from about 70- to 110-cm depth during summer, reflecting the water supply from progressively lower layers for evapotranspiration. Despite its small magnitude, neglecting regional groundwater evaporation in shallow groundwater areas would result in underestimated root-zone soil moisture and hence evapotranspiration by as large as 20% in the dry summer seasons.