Using multiple methods to investigate the effects of land-use changes on groundwater recharge in a semi-arid area

Understanding the applicability and uncertainties of methods for documenting recharge rates in semi-arid areas is important for assessing the successive effects of land-use changes and understanding groundwater systems. This study focuses on estimating groundwater recharge rates and understanding the impacts of land-use changes on recharge rates in a semi-arid area in southeast Australia. Two adjacent catchments were cleared similar to 180 years ago following European settlement, and a eucalypt plantation forest was subsequently established similar to 15 years ago in one of the catchments. Chloride mass balance analysis yields recharge rates of 0.2 to 61.6 mm yr(-1) (typically up to 11.2 mm yr(-1)). The lower of these values probably represents recharge rates prior to land clearing, whereas the higher likely reflects recharge rates following the initial land clearing. The low pre-land-clearing recharge rates are consistent with the presence of old groundwater (residence times up to 24 700 years) and the moderate-to-low hydraulic conductivities (0.31 to 0.002 m d(-1)) of the aquifers. Recharge rates estimated from tritium activities and water table fluctuations reflect those following the initial land clearing. Recharge rates estimated using water table fluctuations (15 to 500 mm yr(-1)) are significantly higher than those estimated using tritium renewal rates (0.01 to 89 mm yr(-1); typically < 14.0 mm yr(-1)) and approach the long-term average annual rainfall (similar to 640 mm yr(-1)). These recharge rates are unrealistic given the estimated evapotranspiration rates of 500 to 600 mm yr(-1) and the preservation of old groundwater in the catchments. It is likely that uncertainties in the specific yield results in the water table fluctuation method significantly overestimating recharge rates, and despite the land-use changes, the present-day recharge rates are relatively modest. These results are ultimately important for assessing the impacts of land-use changes and management of groundwater resources in semi-arid regions in Australia and elsewhere.