Hydrologic Stormwater Modeling in an Arid Environment

The data to develop time variable models in arid, urbanized environments is sparse but necessary to develop accurate time variable watershed models. In this study, stormwater runoff data was collected from individual land use areas in to estimate the flows and volumetric loads of the Santa Monica Bay (SMB) watershed. The time variable model HSPF was used to simulate hydrology and bacterial runoff. The model used a great deal of locally generated data including detailed land use, rainfall, watershed delineation, and stream geometry. The model was calibrated in a mostly open watershed using flow and rainfall data between 1989 and 1998. The model was validated using flow and rainfall data on a second watershed that was mostly urbanized during the same time period. The model validated well after comparing modeled to empirical flow data. The model accurately simulated peak flows, but had difficulty in simulating low flows. Low flows were difficult to simulate in the validation watershed because non-point source flows in urban watersheds are not always rain induced. Moreover, the model had difficulty transitioning from low flow to no flow conditions, as is common in arid undeveloped watersheds. The model accurately simulated the undeveloped watersheds in the North Bay as well as the highly developed in the South Bay.