Evaluation of groundwater replenishment coefficients from the record of a borehole penetrating the unsaturated zone

The coefficients of groundwater replenishment from surficial sources are the most critical parameters in assessing the exploitable yield of an aquifer. So far, the methods used for estimating these coefficients failed to yield plausible results, either because of crude models of the system or because of a simultaneous estimation of many parameters requiring long and diverse data records. This paper proposes a method for obtaining the in situ chronological sequence of the coefficients of replenishment at a given site, independently from the estimates of other parameters. It is based on the profiles of water content and concentrations of tritium and chloride in a single borehole penetrating the unsaturated zone. A combination of these data with the historical record of their inputs at the surface of the ground enables dating of the moisture content in each depth interval. This yields a series of seasonal replenishment coefficients from both natural and anthropogenic sources. The application of the method is illustrated at two sites in the central and southern parts of Israel.