Comparisons and uncertainties of recharge estimates in a temperate alpine catchment

Abstract

Determining recharge rates is critical for understanding and managing groundwater systems. This study compares recharge rates estimated using different methods in a temperate catchment (the Ovens Valley, southeast Australia). A catchment water balance yields recharge rates of 110–410 mm yr−1, with the range reflecting uncertainties in evapotranspiration and the fraction of river water that represents surface runoff. The water table fluctuation method yielded recharge rates of 177–1296 mm yr−1 with a median estimate of 518 mm yr−1. Residual moisture in the unsaturated zone, which lowers the effective specific yield, probably results in this method overestimating recharge rates. Recharge rates estimated from chloride mass balance are also variable (5–712 mm yr−1) with a median value of 285 mm yr−1. Most uncertainties in those estimates arise from uncertainties in the mass of Cl exported directly by surface runoff. Recharge rates calculated from 3H activities using the well-mixed reservoir (renewal rate) model are 11 to 480 mm yr−1 with a median estimate of 159 mm yr−1. The lower recharge rates calculated from 3H may be due to the presence of older water in zones of restricted flow in the aquifers. Additional uncertainties in this technique arise from assumptions in the 3H input function and having to estimate the depth of the active recharge zone at the top of the aquifer. The wide range of recharge rate estimates from methods based on individual groundwater samples may also reflect spatially heterogeneous recharge, which creates difficulties for determining representative recharge rates. The Ovens catchment is typical of many globally in that recharge rates are estimated using infrastructure and data that were not specifically intended for that purpose. The catchment water balance and chloride mass balance are based on abundant commonly-available data and probably yield reasonable estimates of recharge in this and similar temperate catchments.