Separating the impact of climate change from land use change in local and regional groundwater systems

This paper considers the problem of integrating projected future climate change predictions into a catchment modelling framework to determine future impacts on the groundwater resource. Catchment hydrology will also be altered as land managers alter management and land use regimes in response to a changing climate. The response of the groundwater system is further complicated in water-limited environments as plant systems will not respond linearly to changing rainfall, temperature and CO(2) inputs. There is a need to separate the effects on hydrology due to land use change, and those resulting from climate change, to enable adaptation strategies to be proposed that help best manage our groundwater resource. Presented are results from the Catchment Analysis Tool (CAT), a modelling framework which links the daily surface water balance from actual land use management to a calibrated fully-distributed MODFLOW groundwater model. The MODFLOW model, calibrated against historical climate conditions, has been used to assess the groundwater impact under projected changes in rainfall, temperature and solar radiation due to climate change, for the Corangamite catchment in Victoria. Monthly climate change projections provided by the CSIRO climate change model for regional Australia have been converted to daily data using a modified downscaling method. The historic and projected climate data have been used as inputs to a suite of daily biophysical simulation models (CAT) that account for multiple agricultural landscapes, generating mean-annual groundwater recharge estimates which are fed into a steady-state MODFLOW groundwater model. A method proposed to separate the localised groundwater impact of changing land use from the more regional impacts due to climate change is also presented. The following conclusions are drawn from our modelling analyses: The A1F1 climate change scenario resulted in mean annual rainfall for the catchment decreasing by 73 mm and average annual temperature increasing by 1.6 degrees C. Using the current practice scenario, the model predicted that mean annual evapotranspiration would decrease by 7% (39 mm/year) and recharge by 21% (24 mm/year) under climate change; The groundwater resource will be affected by the impact of climate change in Corangamite, especially in the upper reaches; Based on the results shown, a 100 ha tree plantation had a decreased impact on potentiometric head, within 5 km of the plantation due to climate change. The impact, however, was greater between 5 and 10 km; The potential impact of climate change on potentiometric surface can exceed the impact on a tree plantation; and The combined effect of land use and climate change will be large although the response across the landscape is variable.