Evaluating Effectiveness of Best Management Practices to Control Accelerated Sedimentation of the Morro Bay Estuary

The Morro Bay estuary, located on the central Coast of California approximately half way between Los Angeles and San Francisco, is one of the most important wetlands on the west Coast as it supports wide variety of habitats including numerous sensitive and endangered plant and animal species. Various studies have identified accelerated erosion and subsequent sedimentation as a major threat for sustainability of the bay. Watershed disturbances caused by agricultural activities are believed to be one of the major causes of the accelerated erosion and sedimentation. More than 200 conservation practices have been installed in the watershed since the mid-1990 to reduce erosion and sedimentation. This paper will review the implemented BMPs and will evaluate effectiveness of the BMPs using observations and modeling exercise. Streamflow and sediment concentration, measured mainly during the rainy seasons, are available for multiple locations in the watershed. However, the observations are not sufficient in terms of spatial density and data length to evaluate effectiveness of the mitigation measures at various locations in the watershed. It would be daunting in terms of cost to develop an intensive network of monitoring sites that would be needed for reliable management of NPS pollutants. As a result, comprehensive watershed simulation models that integrate watershed and climate characteristics and can estimate pollutant quantity at various locations, and that can also identify source of the contaminants, is emerging as a key component of watershed management. In this regard, a comprehensive watershed simulation model for the Morro Bay watershed has been developed using Soil and Water Assessment Tool (SWAT) to simulate both streamflow and sediment concentration. The observed data was used to improve prediction accuracy of the SWAT model through parameter sensitivity analysis and calibration steps. Parameter sensitivity analysis was performed using step-wise-regression analysis and Morris's one-at-a time (OAT) method. Calibration was performed using four different optimization methods: PEST, Genetic Algorithms, the Shuffled Complex Evolution Algorithm, and Dynamically Dimensioned Search. Relative performance of the sensitivity analysis methods and the calibration algorithms will be discussed in terms of effectiveness and computational efficiency. The developed model was used to evaluate effectiveness of the BMPs implemented in the Morro Bay watershed, and can also be used to prioritize sites where BMPs may be implemented in the future to further improve ecological integrity of the estuary.