Physics-Based Distributed Rainfall-Runoff Modeling of Urbanized Watersheds Revisited with GSSHA

We performed a detailed research comparison in an urban watershed between The U.S. Army Corps of Engineers, Engineer Research and Development Center (ERDC), Gridded Surface/Subsurface Hydrologic Analysis (GSSHA) model and the U.S. Army Corps of Engineers Hydrologic Engineering Center (HEC) HEC-1 watershed model. GSSHA represents a reformulation and enhancement of the CASC2D model. GSSHA has been successfully applied in a number of non-urbanized watersheds. This paper compares the performance of the two-dimensional, distributed parameter model GSSHA against the standard practice conceptual, lumped parameter model HEC-1 for runoff predictions in a small, urbanized watershed. Specific focus is on the calibration and verification results using data from nearly 40 rainfall events in an urbanized watershed. One longstanding criticism of distributed models, such as GSSHA, is that despite more detailed representation of the spatial variability of watershed characteristics and precipitation, their performance has not shown significant improvement over lumped parameter models. Only through marked improvements in runoff prediction can the benefits of the distributed approach be shown superior to the simpler to apply lumped parameter models. Urbanized watersheds provide the best opportunity for this demonstration because they contain a modified drainage network that can have a large effect on the runoff as seen at the watershed outlet. Further, lumped parameter model formulations cannot adequately describe the amount of heterogeneity and the unique spatial organization of an urban watershed. For this reason, accurate urban modeling with widely used standard-practice, conceptual rainfall-runoff models in the United States (i.e. HEC-1, TR20) can be difficult. Statistical analysis of simulation results compared with observed hydrographs clearly demonstrate that GSSHA is superior to standard-practice lumped-conceptual rainfall-runoff models, because of its distributed hydrodynamic formulation. This result has important implications for the development of advanced techniques for runoff modeling and flood prediction in urbanized areas.