The Dandaragan Plateau area of the northern Perth Basin contains a number of regionally important aquifers notably the Yarragadee and Leederville-Parmelia aquifers that support horticulture, mining development and public water supply. This area is characterised by a mild Mediterranean-type climate conducive to groundwater recharge as rainfall (typically 350 – 550 mm / year) occurs primarily during the winter months when evapotranspiration rates are low. Clearing of native vegetation that took place 1950s-1970s has resulted in significantly increased recharge that is manifest in large rises of the regional table (over 20m increase in some areas). However, recent rainfall 2010-2018 is around 80-85% of long term average rainfall and the decline in rainfall since 1990 is consistent with a 2030 dry scenario rainfall trend determined from climate models. The reduction in rainfall has resulted is a significant tapering or even reversal of increasing hydrograph trends.
We present analyses of hydrographs of over 70 long term monitoring bore installed in the 1960s and 1970s. Estimates of pre-clearing recharge using the chloride mass-balance method are made based on chloride concentrations in groundwater and chloride deposition in rainfall (Leany et al. 2011). Additional recharge since land clearing took place is assessed using the hydrograph method (Healy and Cook, 2002). Daily recharge rates are then modelled with the 1D water balance model U3M-1D Class (Vaze et al. 2005) based on unsaturated flow Richards equation. Historical rainfall and reference FAO56 evapotranspiration (EVT) (Allen et al. 1998) from interpolated SILO (Queensland government 2018) data along with soil parameters, rooting depth profiles and leaf area index for the crop growing season are key model parameters. The 1D recharge model is calibrated and verified over different portions of observed hydrographs. Simulations with the calibrated 1D recharge model estimate future recharge for future wet, median and dry future scenarios based on climate model data.
A useful result from the recharge modelling is estimation of the rainfall recharge relationship which shows approximately that a threshold amount of annual rainfall of around 300mm is required before any significant recharge in any particular year. The modelling also highlighted the importance of the timing of rainfall to generate deep infiltration and recharge especially at the end of the winter rainfall season when the soil profile is saturated. Long term dry climate scenarios indicate large reductions in the amount of recharge which significantly impacts the long-term sustainable yield of groundwater resources.