Impact of Antecedent Wetness and Rainfall Spatial Variability on Design Floods
McMahon, Rachael Nicole
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In the absence of long hydrologic records, water engineering design application rely primarily on the concept of design storms and subsequently design floods. However, despite the long-lasting use of these approaches there are several aspects that merit careful consideration and possibly revision. Representation of spatial variability of rainfall extremes in design storms has relied primarily on the use of area reduction factors that while useful, they do not represent realistic patterns of flood inducing storms. Furthermore, the impact of antecedent wetness on the transformation of design storms to design floods is often neglected. In this work we carry out a numerical experiment to investigate and quantify the impact of rainfall spatial variability and antecedent wetness on design floods. The study is focused on a medium size (~1000km2) watershed in Colorado. We utilize the NEXRAD stage IV radar-rainfall record to derive realistic storm patterns for the September 2013 Colorado flood event and then use the rainfall data to develop additional precipitation patterns that are commonly used in engineering design methodology. Flood responses for the various design storm methods are simulated using the WRF-Hydro distributed hydrologic model by considering also different initial soil moisture conditions. Results reveal that both antecedent wetness and spatial rainfall pattern exert major control on simulated flood response. These findings highlight the complexity in linking precipitation and flood quantiles and suggest that information on the soil moisture state and spatial rainfall pattern should be explicitly integrated in the estimation of design floods.