TEMPORAL AND SPATIAL VARIABILITY OF GROUNDWATER RECHARGE: A GIS BASED STUDY IN A SUBWATERSHED IN DEKALB COUNTY, IL

Effective groundwater management requires the reliable calculation of historical groundwater balances at local, subwatershed scales to estimate groundwater recharge. Using a simple soil-water balance model combined with Geographic Information System (GIS) methods, annual recharge rates in a subwatershed of the Kishwaukee River in DeKalb County, IL, an area of rapid urbanization, population growth, and increasing groundwater demands, were estimated for three different years (1996, 2000 and 2005). These years were chosen to represent both extreme and average weather conditions. The input parameters include basic soil characteristics, land cover, daily precipitation, potential evapotranspiration and plant moisture availability; the calculated output includes daily actual evapotranspiration, initial abstraction, run-off and areal groundwater recharge rates. Actual ET was calculated using a two-parameter model (Zhang et al. 2001) and the run-off was calculated using the NRCS SCS curve number method. As spatially variable phenomena, the hydrologic parameters are best characterized using GIS techniques; these are also being used to identify combinations of soil type and land use for recharge estimates. To make the calculations more reliable and efficient, the model is automated using ArcGIS 9.1 model building tool and Python script. The calculated areal recharge is, as expected, a function of soil type and land-cover. In forest or agricultural areas, recharge rates are high on sandy soil, low on clayey loamy soil, and also low on urban land-cover. The spatial variation is large, with calculated recharge rates ranging from 0.2 to 13 inches per year. Temporal variation results from changing weather parameters like precipitation and temperature. The recharge estimates determined by GIS are being used as an input parameter for a groundwater flow model of the area.