Future Storm Frequency and Runoff in Small US Mid-Atlantic Watersheds Evaluated Using Capture Depth

Abstract

Due to climate change and urbanization, runoff events will become more frequent, resulting in increased potential for flooding and soil erosion. To understand the hydrologic impacts of various climate change and urbanization scenarios in the state of Maryland, this study assesses the frequency, intensity, and associated runoff conditions of index storm events as hydrologic indicators for stormwater management. The analyzed events are defined as capture depth, that is, the depth of event precipitation that accounts for a specified fraction (85%, 90%, 95%, and 99%) of total rainfall when all event depths are ranked and cumulated. Four representative watersheds (area of approximately 3 km2) are analyzed across four populous counties in Maryland. A statistically significant increasing trend in the frequency of these events is observed during the historical period 1981-2015. For the future period 2016-2035, an ensemble of bias-corrected Coupled Model Intercomparison Project (CMIP5) models shows an increase of 1%-5% in mean precipitation. Two different downscaling methods are applied to generate time series of future event precipitation for the study watersheds from the CMIP models: change factor (CF) and multivariate adaptive constructed analogs (MACA). Modest increases in the frequency of events in the 85%-99% range of capture depth are observed across all counties. Runoff associated with events greater than the 85% capture depth is computed using the Natural Resources Conservation Service curve number method. Both the CF and MACA projected future precipitation time series are used to calculate the response to 24-h precipitation under two scenarios: (1) climate change, and (2) climate change plus urbanization. Runoff frequency distribution modeled under both scenarios indicate that climate change is more influential than urbanization in this region. In addition, storage volumes for enhanced bioretention using CF-based future climate projections show that both climate change and urbanization have similar potential to affect stormwater management costs.