Runoff Ratios for Five Small Urban Karst Watersheds under Different Levels of Imperviousness

Continuous stormwater runoff data were collected for three years from five urban karst watersheds at seven monitoring stations in the limestone geologic setting of the Nittany Valley in Centre County, Pennsylvania. The watersheds encompassed a range of imperviousness (23.4% to 67.3%) and different land uses (including commercial development, high density campus and student housing, single family residential, big box and strip mall development, and farm fields). The runoff ratio (ratio of volume of stormwater runoff to volume of rainfall) was calculated for each watershed. In general, the more impervious watersheds produced higher peak rates, runoff volumes, and runoff ratios than the less impervious watersheds, as would be expected. The median runoff ratio (based on more than 60 events) for the most impervious watershed (67.3% impervious) was 0.28, whereas the median runoff ratio for the least impervious watershed with natural karst drainageway features (23.4% impervious) was 0.02. The runoff ratios for the study karst watersheds were compared with runoff ratios as a function of imperviousness (0.5 to 0.62) from the EPA National Urban Runoff Program (NURP). The runoff ratios for these karst watersheds (median values ranging from 0.02 to 0.284) were significantly less (with 95% confidence) than what would be expected based on the percentage of impervious surface. If used in design-analysis calculations, the runoff ratios based on the NURP data would overpredict runoff in these karst watersheds by 203% to 14,000% based on the observations from the study period. The small runoff ratio values found in this study can be used to inform stormwater modeling and runoff calculations in karst settings.