Estimating travel times of nitrate from the root zone to the water table at the basin scale: Tulare Lake Basin and Salinas Valley

Best Management Practices (BMPs) in agriculture are developed for a number of reasons, often with the intent of minimizing the release of chemical pollutants to surface and ground water. There is a lag time from when a BMP is enacted, and when its effects can be seen at a particular location. Nitrate, being the most pervasive pollutant in agricultural areas of California, has been extensively researched regarding the improved efficiency and management of its use. The goal of this investigation is to determine areas within the Tulare Lake Basin and Salinas Valley where the effect of BMPs will be observed the soonest at the water table, and to identify which areas will experience significant lag. Although heterogeneity can cause preferential flow through the vadose zone, resolving detailed soil structure for 2D and 3D simulations is not feasible at the scale of 1000's of sq. miles. In light of this, three maps were created based on three homogeneous soil types: sand, loam, and clay soil, representing the quickest, intermediate, and slowest probable travel times, respectively. HYDRUS 1D was used to model travel times to the water table by specifying annual leachate fluxes and depth to the water table. Annual fluxes of agricultural return water were determined by mass balance using the differences between calculated evapotranspiration from a field and the amount of water applied through natural precipitation and irrigation (including various irrigation technologies and their associated efficiencies). The result of this modeling effort is a regional scale map of the spatial distribution of travel times to the water table. This provides a helpful tool for regional planners, distinguishing where adjusted BMPs can have a relatively quick impact to water quality at the water table compared to areas which will likely experience longer time lags to show a response.