Future storm frequency and runoff in small Mid-Atlantic watersheds, evaluated using capture depth

Authors: KHAN, I.M.P - University Of Maryland; Moglen, Glenn; HABACEK, K. - University Of Maryland; BRUBAKER, K.L. - University Of Maryland

Submitted to: Journal of Sustainable Water in the Built Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/29/2019
Publication Date: 4/25/2019
Citation: Khan, I., Moglen, G.E., Habacek, K., Brubaker, K. 2019. Future storm frequency and runoff in small Mid-Atlantic watersheds, evaluated using capture depth. Journal of Sustainable Water in the Built Environment. https://doi.org/10.1061/JSWBAY.0000879.
DOI: https://doi.org/10.1061/JSWBAY.0000879

Interpretive Summary: Stormwater best management practices (BMPs) are efforts at mitigating the negative consequences of storm runoff. Such efforts range from the planting of cover crops to prevent erosion and reduce nutrient runoff, to structural solutions such as infiltration devices and detention ponds. This paper uses “capture depth” as a measure for identifying storm events that are simultaneously large enough to cause significant negative impacts, and frequent enough that they may happen several times a year. Capture depths associated with storms that exceed the 85%, 90%, 95%, and 99% thresholds based on current annual rainfall characteristics are examined. Trends in projected future rainfall from multiple climate models were examined and significant trends in the number of threshold-exceeding events were determined in this study. Land use change, as a driver of trends in runoff behavior, was also examined both with current rainfall and projected future rainfall. Findings indicate that future rainfall is possibly more influential than land use change in driving trends in future runoff. Economic consequences of both changing rainfall and land use are examined in the context of a specific BMP: dry extended detention ponds. Findings indicate that changing rainfall behavior and land use change have similar potential to influence costs associated with these changes and this specific BMP.

Technical Abstract: Due to climate change and urbanization, small scale runoff events will become more frequent, resulting in increased potential for flooding and soil erosion. To understand the hydrological impacts of various climate change and urbanization scenarios in the state of Maryland, we assess the frequency, intensity and associated runoff conditions of index storm events as hydrological 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, 99%) of total rainfall when all event depths are ranked and cumulated. Four representative watersheds (area ~3km2) are analyzed. 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 methods are applied to generate time series of future precipitation for the study watersheds from the CMIP models: a simple, Change Factor (CF) downscaling method and the Multivariate Adaptive Constructed Analogs (MACA) downscaling method. Modest increases in the frequency of events in the range 85-99% 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-hour precipitation under two scenarios: climate change, and climate change plus urbanization. Runoff frequency distributions obtained under both scenarios indicate that climate change is more influential than urbanization in this region. In addition, storage volumes for dry extended detention ponds using CF-based climate projections of future period show that both climate change and urbanization have similar potential to affect stormwater management costs.