The Distributed Recharge Mechanism in the Australian Arid Zone

Abstract

AbstractWe developed a model to describe fluctuations in historical records of well water levels in the Australian arid zone in order to determine the average recharge rate and specific yield of the phreatic aquifer. The model was used to explore the degree to which variations in solute concentrations are preserved in the deep unsaturated zone during convection to the water table during recharge of rainwater. An examination of historical water levels in wells gives support to the hypothesis (obtained from an earlier study on NO3 contamination of groundwater) that distributed recharge is the primary source of groundwater renewal. Detailed comparison of the hydrograph records suggests a nonlinear response to rainfall, with an “initial loss” of approximately 130 mm mo−1 (five times the mean rainfall). The best estimate of the long‐term recharge rate is about 20 mm yr−1, although the last 10 yr appear to have been somewhat wetter than usual, due mainly to a very large rainfall event in 1989. This recharge mechanism implies that historical environmental tracer signals in recharge waters moving through deep unsaturated zones in unstructured soils will be largely unaffected, when recharge rates are low, by dispersion or even diffusion below a depth that depends on the variability of the rainfall.