Hydrologic Model Development of the Sacramento River Watershed to Support TMDL Development

The Sacramento River watershed flows in portions of Oregon and California, encompassing an area of approximately 27,908 square miles. Many streams in the watershed are included on California's 303(d) list of impaired waters as a result of multiple pollutants, most notably pesticides and metals. In response to the listings, the Central Valley Regional Water Quality Control Board (RWQCB) has initiated the process of calculating total maximum daily loads (TMDLs) for these waterbodies. The development of TMDLs will require an assessment of the streams' assimilative capacity, critical conditions, and other considerations. Although water quality data are available, many stream segments in the watershed have little or no flow data to support the TMDL analyses. To address this major data gap, Tetra Tech has supported the RWQCB and USEPA in development of a hydrology model of the Sacramento River watershed. Once configured and calibrated, the model can be used by the RWQCB for use in predicting flows in ungaged streams to support TMDL calculations.Due to the unique hydraulic controls and management practices in the watershed, special consideration was made regarding model capabilities in simulating not only hydrology, but also in-river hydrodynamics impacted by backwater and diversions to agricultural areas for irrigation. The resulting modeling system has been divided into two components representative of the processes essential for accurately modeling the system. The first component consists of a watershed model that predicts storm water runoff associated with rainfall events. The Loading Simulation Program C++ (LSPC) is used for simulation of watershed hydrology. The second component includes a hydrodynamic model to simulate the complex flow and pollutant transport patterns of the lower portions of the Sacramento River and tributaries affected by backwater. The Environmental Fluids Dynamic Code (EFDC) is used for the hydrodynamic receiving water model of these river segments.Modeling the Sacramento River watershed presents a challenge because of the system's unique hydraulic features that include irrigation diversions, flood control levees and bypass channels, storage of headwaters in reservoirs, and various management practices that potentially enhance or impede flow. In addition, to assist in TMDL development and to provide decision support for watershed management, the model will be used to simulate various scenarios and may require future modifications to address specific management and environmental factors. Such scenarios may result from the augmentation of input data to be collected in ensuing monitoring efforts, future implementation of various management strategies or BMPs, adaptation and linkage to additional models developed in subsequent projects, or upgrade for simulation of water quality. Therefore, model flexibility was a key attribute for model selection.