The nature of the recent rainfall decrease in the vicinity of Melbourne, southeastern Australia, and its impact on soil water balance and groundwater recharge

Rainfall at three stations in the vicinity of Melbourne, southeastern Australia, was analysed to determine the nature of the approximately 20 per cent reduction in annual rainfall observed from 1997-2010 (the Big Dry ). No change has occurred in the amount of rainfall per rain day, rather a decrease in the number of rain days has occurred, especially those with higher rainfalls. These are the rain days which contribute most to significant groundwater recharge and streamflow. A soil water balance model was used to investigate the effect of this change in rainfall on groundwater recharge. The increase in time between rain days, associated with the reduced number of rain days, allows for greater drying of the soil before the next rain day, thereby reducing the water available for groundwater recharge. This leads to a disproportionately greater reduction in groundwater recharge compared to the actual reduction in rainfall, consistent with observations.