Abstract
Abstract. The role of rainfall space–time structure, as well as its complex interactions with land surface properties, in flood response remains an open research issue. This study contributes to this understanding, specifically for small (<15 km2) urban watersheds. Using a flood frequency analysis framework that combines stochastic storm transposition (SST)-based rainfall scenarios with the physically based distributed Gridded Surface Subsurface Hydrologic Analysis (GSSHA) model, we examine the role of rainfall spatial and temporal variability in flood frequency across drainage basin scales in the highly urbanized Dead Run watershed (14.3 km2), Maryland, USA. The results show the complexities of flood response within several subwatersheds for both short (<50 years) and long (>100 years) rainfall return periods. The impact of impervious area on flood response decreases with increasing rainfall return period. For extreme storms, the maximum discharge is closely linked to the spatial structure of rainfall, especially storm core spatial coverage. The spatial heterogeneity of rainfall increases flood peak magnitudes by 50 % on average at the watershed outlet and its subwatersheds for both small and large return periods. The framework of SST–GSSHA-coupled frequency analysis also highlights the fact that spatially distributed rainfall scenarios are needed in quick-response flood frequency, even for relatively small basin scales.
Highlights
Rainfall spatiotemporal structure plays an important role in flood generation in urban watersheds (Saghafian et al, 1995; Smith et al, 2005a; Emmanuel et al, 2012; Nikolopoulos et al, 2014)
This paper addresses the problem of the impacts of shortduration rainfall variability on hydrologic response in the small urbanized watershed
The main findings are as follows: 1. The flood frequency distributions for subwatersheds within the highly urbanized 14.3 km2 Dead Run watershed demonstrate the complexities of flood response for both short and long rainfall return periods
Summary
Rainfall spatiotemporal structure plays an important role in flood generation in urban watersheds (Saghafian et al, 1995; Smith et al, 2005a; Emmanuel et al, 2012; Nikolopoulos et al, 2014). The idealized assumptions include idealized rainfall temporal structure, uniformed spatial distribution and 1 : 1 rainfall–flood return period equivalence (see Wright et al, 2013, 2017; Zhou et al, 2019, and references therein). These assumptions ignore the interaction between the spatiotemporal structure of rainfall and flood response, which increases the uncertainty of frequency estimations. We characterize the spatial and temporal features of rainfall events under different return periods and examine their roles in determining flood frequency in small urban watersheds.
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