Quantifying climate contributions to changes in groundwater discharge for headwater catchments in a major Australian basin

Groundwater is experiencing a higher risk of aquifer depletion due to longer drought duration and increasing water demand induced by climate change. The climate impacts on groundwater can be propagated to changes in groundwater discharge to rivers, which will deeply alter the connection between groundwater and surface water and reshape the fundamental functions of the river system especially in maintaining environmental flows. In synchronization with the drying and warming climate, groundwater discharges estimated using digital filtering approaches are found to have experienced significant reduction since the 1990s for all our studied headwater catchments in the Murrumbidgee portion of the Murray-Darling Basin. The linkage between precipitation and groundwater discharge is demonstrated to be seasonally dependent. For most of the studied catchments, the dominant precipitation metrics affecting groundwater discharge are the winter precipitation followed by autumn and spring precipitation. Multivariate nonlinear regression modelling suggests that the relationship between groundwater discharge and the dominant climate variables can be represented statistically by a power law. The individual contribution of each dominant climate variable quantified based on the concept of elasticity shows that the decrease in precipitation outweighs the increase in potential evapotranspiration in contributing to the reduction in groundwater discharge. The autumn precipitation accounts for a larger proportion of the changes in groundwater discharge in all studied catchments because of its relatively higher elasticity and change rate. The reduction in groundwater discharge since the mid-late 1990s in the headwater catchments can largely (estimated here at >75%) be attributed to climate factors.The research of the first author is supported by the Australian Gov- ernment Research Training Program (AGRTP)