INCORPORATION OF CHLORIDE MASS BALANCE TO IMPROVE ESTIMATES OF EVAPOTRANSPIRATION, RECHARGE, AND RUNOFF

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