22. Hydrology and Water Quality Modeling of the Los Angeles River 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 The purpose of the Green Visions Plan project watershed health assessments is to support and inform region-wide planning efforts from the perspective of habitat conservation, water protection, and recreational opportunities in southern California. In this report, hydrologic models of the Green Vision's 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 MIKE BASIN, 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 and meteorological data were used to develop the model segmentation and input, and detailed streamflow data were selected to conduct model calibration and validation over a nine year period (10/1996 – 9/2005). Both quantitative and qualitative comparisons were developed to support the model performance evaluation effort. The calibration and validation were performed at 14 stream locations throughout the watershed, for annual runoff, daily and monthly stream flow, water balance components, and annul water quality. The results, based on the graphic comparison and error analyses described herein, demonstrated a fair to good representation of the observed flow data. As shown in Figures A-5 through A-11, the model simulated the total water volumes fairly well for the 10 validation sites. Very good validation results were achieved for simulating the 90th percentile high flows while the 10th percentile low flows were poorly simulated with over-predictions at all sites. The water quality simulations were not as satisfactory as the flow simulations in reproducing the observed sample concentrations. Many predictions of constituent concentrations fell outside the range of acceptable values that was used for the water quality assessment. Graphically, some sample concentrations were captured while others were missed in the pollutographs and it did not always predict …