Controls on the Spatial Distribution of Ground-water Recharge in Washington County, Wisconsin

Groundwater recharge has been measured indirectly by separating baseflow from streamflow hydrographs in 11 small watersheds in southeastern Wisconsin. Over a two year study period, observed recharge rates at these sites and additional test locations ranged from 3 to 25 cm/yr. The majority of observations fell in a smaller range, from 10 to 14 cm/yr.

The spatial distribution of recharge (R) was linked to independent topographic, hydrogeologic, land use and climatic conditions. The latter was eliminated by normalizing recharge to annual precipitation (P), as RIP. Normalized recharge for the study area ranged between 0.03 and 0.26 (3 to 26% of precipitation recharges the groundwater). The normalized values were then shown to vary directly with soil permeability, length of overland flow and the portion of the land surface remaining natural. It varies inversely with hillslope, depth to water table and amount of developed land use. The relation developed to predict RIP from these independent variables accounts for 91 % of its spatial variation in Year 1 of the study, the year from which the relation was developed.

The relation was able to predict RIP at independent test sites for the same year to within ± 19%. The predictions were most accurate for drainage areas in the range of 3 to 225 km2• Use of the relation for a second year of record required adjusting it for climatic differences between the years. After those adjustments, prediction was accurate to within± 30% in the study area sites and± 40% in outside test sites. The reduction in accuracy suggests that the climatic corrections need further refinement. All the testing has been done in southeastern Wisconsin, so it is not known whether the relation can be used elsewhere.