Impacts of Urbanization on Base Flow and Recharge Rates, Northeastern Illinois: Summary of Year 1 Activities

During year one of a two-year project to investigate the impacts of urbanization on base flow and ground-water recharge rates in northeastern Illinois, three gaged watersheds in urbanized areas of northeastern Illinois, and one watershed located in rural northwestern Illinois, have been selected for study. The gages have a common period of record extending from October 1952 through the present, a period during which the northeastern Illinois watersheds underwent substantial urbanization. Mean daily discharge data from the gages have been analyzed using an automated hydrograph separation technique, and monthly estimates of mean total discharge, base flow, and direct runoff have been calculated. Spearman rank correlation coefficients indicate a stronger correlation between precipitation and total discharge, base flow, and direct runoff in the northeastern Illinois watersheds than in the rural watershed. Smoothed time-series plots of total discharge, base flow, and direct runoff in the urban watersheds are less consistent with precipitation than similar plots constructed from the rural watershed data. The trends indicate that rates of direct runoff have overtaken rates of base flow in two of the three northeastern Illinois watersheds, but in one of these watersheds, this relationship probably reflects the cessation of effluent discharges to the stream. In general, double-mass curve analysis suggests that, relative to the rural watershed, base flow in the urban watersheds has proportionally decreased, and direct runoff has proportionally increased. The trends suggested by the smoothed time-series plots and the double-mass curves are consistent with a conceptual model of the northeastern Illinois watersheds in which sewering and impervious surfaces have reduced infiltration, and thence ground-water recharge and base flow, in the watersheds.Ope