Assessment of salt and nitrate sources and loading implications using a coupled surface water/groundwater model: a Central Valley example

A study is underway as a key initial step in the effort by the Central Valley Salinity Coalition (CVSC) to help develop a Basin Plan amendment to address the issue of salt and nutrient management in the Central Valley. The overall objectives of the study are to develop and document procedures and methodologies to quantify the significant salt and nitrate sources in the Central Valley and to pilot those procedures in selected areas to validate the appropriateness and region-wide applicability of the procedures. The study employs the use of the Watershed Analysis Risk Management Framework (WARMF) watershed model in coordination with the finite difference groundwater flow model, MODFLOW, to evaluate salt and nitrate mass loading in three pilot study areas. Previously developed MODFLOW groundwater flow models for the Modesto and the Tule River pilot study areas will be used to provide information about recharge rates and groundwater pumpage to the WARMF model; salt and nitrate mass loads developed from the WARMF model will be shared with the groundwater models. MODFLOW with the particle tracking program, MODPATH, will then be used to provide a map of the distribution of groundwater originating from different sources using backward and forward particle tracking techniques. In a third pilot study area in Yolo County, it is planned that the recently released USGS Central Valley Hydrologic Model (CVHM), which uses advanced components of MODFLOW (MODFLOW-2000, or MF2K, including the Farm Process) will provide data such as for groundwater pumpage, deep percolation, and overland farm runoff for inputs to the WARMF domain in this study area.