QUANTIFYING THE EFFECTS OF URBANIZATION ON TWO WATERSHEDS: HOW DOES DISCHARGE SCALE WITH DRAINAGE AREA IN AN URBAN AREA?

The negative influence of urbanization on streams is generally assumed, but poorly quantified. We select two contiguous watersheds in east-central Pennsylvania, the Little Lehigh River and Sacony Creek, with similar watershed area, annual precipitation, relief, and underlying substrate, but different amounts of urbanized land to test the dependence of discharge on drainage area. The Little Lehigh watershed has been rapidly urbanizing and now contains over 20% urbanized land, while the Sacony Creek watershed is mostly rural and has less than 5% urbanized land. Pressure sensors were installed in the trunk channels of each stream allowing a rating curve to be developed at each site. The sites were chosen to maximize the variation in upstream drainage area (A) in order to explore its influence on discharge (Q). Both base flow and peak discharges from storm events over several seasons were calculated for the two streams. Discharge grows as a power function with respect to area following the general equation Q = kA^c, where k is a crude measure of stream baseflow, and c is the power dependency. For many watersheds, discharge scales linearly or nearly linearly with drainage area (c ~1). This simple relationship is assumed and enjoys widespread application in research including the stream power law for bedrock erosion. In our small watersheds, c varies from 1 to nearly 2, an effect that we attribute to degree of urbanization. To test this interpretation, we assemble several decades of USGS gage data on other, nearby watersheds to calculate c through time. Our results show systematic drift in the value of c that appears to track with changes in landuse, most notably, the increase in urbanization in these watersheds. Although in its initial phases, these trends in how discharge scales with drainage basin area could have major sedimentologic, ecologic, and geomorphic impacts on watersheds undergoing land use changes.