Managing Uncertainty in Runoff Estimation with the US Environmental Protection Agency National Stormwater Calculator

The U.S. Environmental Protection Agency National Stormwater Calculator (NSWC) simplifies the task of estimating runoff through a straightforward simulation process based on the EPA Stormwater Management Model. The NSWC accesses localized climate and soil hydrology data, and options to experiment with low-impact development (LID) features for parcels up to 5ha in size. We discuss how the NSWC treats the urban hydrologic cycle and focus on the estimation uncertainty in soil hydrology and its impact on runoff simulation by comparing field-measured soil hydrologic data from 12 cities to corresponding NSWC estimates in three case studies. The default NSWC hydraulic conductivity is 10.1mm/h, which underestimates conductivity measurements for New Orleans, Louisiana (95 +/- 27mm/h) and overestimates that for Omaha, Nebraska (3.0 +/- 1.0mm/h). Across all cities, the NSWC prediction, on average, underestimated hydraulic conductivity by 10.5mm/h compared to corresponding measured values. In evaluating how LID interact with soil hydrology and runoff response, we found direct hydrologic interaction with pre-existing soil shows high sensitivity in runoff prediction, whereas LID isolated from soils show less impact. Simulations with LID on higher permeability soils indicate that nearly all of pre-LID runoff is treated; while features interacting with less-permeable soils treat only 50%. We highlight the NSWC as a screening-level tool for site runoff dynamics and its suitability in stormwater management.