Spectrum of storm event hydrologic response in urban watersheds

Key Points Peak discharge and runoff ratio distributions vary greatly between urban basins Stormwater detention infrastructure increases response times and runoff ratios Spatial and temporal properties of flood-producing rainfall vary over the basins We seek to improve scientific understanding of urban storm event hydrologic response through analyses of rainfall and discharge data for the Baltimore metropolitan region. High-resolution data, 1 km ² and 15 min radar rainfall and 1 to 5 min discharge, provide the detail necessary to accurately characterize storm event hydrologic response in small urban basins. We examine flood-producing rainfall properties and storm event hydrologic response for nine small watersheds in the Baltimore region including seven urbanized basins, a forested basin, and an agricultural basin. We find expected contrasts in flood peak distributions and storm event runoff production between the urban and nonurban watersheds, but we also find a spectrum of storm event hydrologic response among the urban watersheds. Moores Run and Dead Run are end-members of this urban spectrum, with Moores Run producing anomalously large flood peak magnitudes and Dead Run producing anomalously large storm event runoff ratios. Analyses show that runoff production and timing of hydrologic response are linked to stormwater management infrastructure and play a central role in the spectrum of storm event response. Detention basins in these watersheds appear to operate as intended by stormwater legislation to lower peak discharges but not runoff volumes. Antecedent moisture does not appear to significantly impact storm event hydrologic response in the urban or nonurban basins. The rainfall climatology of flood-producing storms varies from urban to nonurban watersheds with urban watershed flood frequency displaying a pronounced warm season maximum, highlighting the central role of warm season thunderstorm systems for urban flooding in Baltimore.