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INCORPORATING TIME VARIABLE RUNOFF MEASUREMENTS IN WATERSHED MODELING

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Time variable models of arid, urbanized watersheds are rare, but are becoming necessary to estimate emissions from unmonitored areas and predict the effectiveness of various management scenarios. In this study, a time variable model is developed to estimate the flows and volumetric loads of the Santa Monica Bay (SMB) watershed.

The time variable model HSPF using 15 min increments was used in this study. 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 1988 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. Except for a few site-specific reasons, the model was unable to predict low flows during dry weather because flows in urban watersheds are not always rain induced. Moreover, the model had difficulty transitioning from low flows to no flow conditions.

Water quality has been extensively monitored in southern California. Over 1,700 stormwater sampling events have been monitored by local agencies between 1993 and 1999. The majority of the data is a single grab sample during an event. We monitored runoff from six land use categories (agriculture, commercial, high density residential, low density residential, open, and transportation). Samples were taken every 15 minutes throughout the course of an event. The model was calibrated in each catchment and bacteria runoff parameters calibrated to measured pollutographs.

Applying the coefficients in larger watersheds with mixed land uses validated the model. The model validated well when comparing the measured and modeled pollutographs. The water quality component of the model also had some difficulties with intermittent streams during the transition from flowing to dry but these were resolved when a reasonable minimum flow was assumed for each watershed.

The model accurately simulated the undeveloped watersheds in the North Bay as well as the highly developed in the South Bay. It also represented the bacteria runoff patterns well in an arid environment. The most problematic land use calibration was “open”. This resulted from several factors. First, our sampling of the land use was hindered by lack of flow. Second, open land use concentrations can vary significantly among sites.

Therefore, the open land use incorporated all data from the region. An open catchment was simulated for two years and the flow weighted mean concentration calculated. Buildup and maximum concentration were adjusted until the geometric mean and the flow weighted mean were comparable.
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Document Type: Research Article

Publication date: 2002-01-01

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