COMPARISON OF GROUND-WATER RECHARGE RATES DETERMINED WITH TWO GIS BASED METHODOLOGIES

Net ground-water recharge (infiltration minus evapotranspiration) has been determined two different ways for 66 individual subdivisions of 7 watersheds in southeastern Wisconsin. Stream baseflow separation and precipitation-runoff distribution modeling produce excellent agreement, showing recharge rates range from nearly 0 cm/yr (in urban areas) to over 30 cm/yr. Well hydrograph analysis, an independent third approach, confirms that the two indirect methods produce a reasonable value.

Baseflow separation has been used to generate an empirical equation linking recharge rate to common watershed properties available on GIS data bases (topography, land cover, surficial geology). The distribution model (PRMS of the USGS) distributes precipitation among runoff, evapotranspiration and infiltration. The latter is then further distributed among soil, unsaturates and saturated zones via flux equations. PRMS has been adapted for this study by making these equations Darcian analogues with a conductance term set equivalent to relevant hydrogeological properties, all but one of which are obtainable from GIS sources.

Results from the two methods show that soil permeability and topographic slope are the two most important conditions controlling recharge rates. Land cover, depth to the water table, azimuthal aspect of the subwatershed and other factors also contribute to the spatial variability of recharge. The comparability of the two methods allows them to be used in tandem to calculate recharge. PRMS is applicable only to gaged watersheds, but it can also show the seasonal variation of recharge. The baseflow-derived empirical equation allows calculation of recharges in ungaged watersheds. Combination with GIS makes both methods readily applicable to other areas.