19 . Hydrology and Water Quality Modeling of the Santa Monica Bay Watershed

The mission of the Green Visions Plan for 21st Century Southern California is to offer a guide to habitat conservation, watershed health and recreational open space for the Los Angeles metropolitan region. The Plan will also provide decision support tools to nurture a living green matrix for southern California. Our goals are to protect and restore natural areas, restore natural hydrological function, promote equitable access to open space, and maximize support via multiple-use facilities. The Plan is a joint venture between the university of Southern California and the San Gabriel and The " Green Visions Plan for Twenty-first Century Southern California " project's watershed health assessment seeks to support and inform region-wide planning efforts that promote habitat conservation, water quality protection, and the creation of new recreational opportunities. In this report, hy-drologic models of the Green Vision Plan watersheds were developed for use as a tool for watershed planning, resource assessment, and ultimately, water quality management purposes. The modeling package selected for this application is the Danish Hydrology Institute's (DHI) MIKE BASIN. MIKE BASIN is a watershed model of hydrology and water quality, which includes modeling of both land surface and subsurface hydrologic and water quality processes. It was used to evaluate the current baseline hydrologic conditions and water quality and pollutant loadings in the GVP's five 8-digit HUC watersheds, namely, the Los Angeles River, meteorological data were used to develop the model seg-mentation and input, and detailed streamflow data were selected to conduct model calibration and validation over a nine year period (10/1996—09/2005). Both quantitative and qualitative comparisons were developed to support the model performance evaluation effort. The calibration and validation results, based on the graphic comparison and error analyses described herein, demonstrate a fair to good representation of the observed flow data. Statistical comparisons and model performance evaluation were performed at three stream locations throughout the watershed, for annual runoff, daily and monthly streamflow, water balance components, and annul water quality. These comparisons demonstrate conclusively that the model is a good representation of the water balance and hydrology of the Topanga Canyon and Malibu Creek subwatersheds. The model has demonstrated consistently fair to good simulations of total water volume and high winter and spring flow conditions, but poor modeling of summer low flows for the minimally developed subwatersheds draining into Santa Monica Bay. The water quality simulations did not match the mean concentrations and temporal variations in concentrations …