Incorporation of chloride mass balance to improve estimates of evapotranspiration, recharge, and runoff

A method is presented whereby measured soil water chloride concentrations and long-term precipitation and air temperature profiles are interpreted to provide temporal estimates of evapotranspiration, recharge, and runoff. Applying the chloride mass balance technique to soil water chloride profiles improves the boundary conditions associated with the long-term mean recharge rate. Temporal estimates of evapotranspiration and runoff are calculated from precipitation and air temperature data. Next, these estimates and measured precipitation are used as inputs in an unsaturated groundwater model to estimate temporal recharge, which is subsequently compared to the long-term mean recharge rate calculated from the chloride profiles. Finally, the evapotranspiration and runoff components of the model are scaled such that the modeled recharge rate is similar to the long-term mean recharge rate. This method improves the chloride mass balance method, which up to now only provides long-term mean recharge. Additionally, the method allows initial estimates of evapotranspiration and runoff to be scaled such that the resulting estimates of evapotranspiration and runoff are consistent with both chloride mass balance and water mass balance. Although direct methods to measure evapotranspiration, recharge, and runoff are attractive, they are not always reasonable due to the expense of collecting data over long time periods. In contrast, this method obtains its required input from basic meteorological data and soil cores collected at a single point in time.