Assessing the Impact of Spatial Scaling on Empirical Runoff Ratio Models within a Heterogeneous Suburbanizing Watershed in Central Indiana

Suburbanized watersheds are characterized by a spatially complex mosaic of fragmented impervious and vegetated surfaces. The heterogeneous nature of land cover in and the variety of storm routing structures that accompany suburban development substantially modify surface hydrological dynamics within suburban watersheds. The hydrological consequences of land use and land cover change create a pressing issue for the management of water resources within developing watersheds. This research examines the hydrological impacts of recent population growth and accompanying suburban development within the Jack's Defeat Creek watershed in Ellettsville, IN, which is a 40 km2 basin that has experienced an approximate doubling in population in the last 25 years. Event-based, whole-basin runoff responses are determined from streamflow and precipitation data collected during 2005 and 2006 under both wet and dry antecedent conditions. Observed runoff responses are compared to multi-scale predictions of runoff ratios derived from the Soil Conservation Service Curve Number, which is an empirical model commonly used by municipal planning agencies to generate runoff estimates based largely on characterization of land cover and soil type. The comparison of observed whole-basin runoff response to predictions of the SCS Curve Number derived from a range of spatial scales addresses both (1) the accuracy of the Curve Number method as a predictor of runoff response in heterogeneously impervious landscapes as well as (2) the spatial scale at which the runoff estimates from empirical approaches best match the observed data under varying antecedent moisture conditions. These results will provide guidance regarding the best practices for employing empirical rainfall-runoff relationships to predict storm runoff responses in rapidly urbanizing watersheds.