ARID ZONE RECHARGE MECHANISMS AND THE TIME TO REACH A NEW HYDRAULIC EQUILIBRIUM

A conceptual understanding of different recharge processes is beginning to emerge for the arid western margin of the Great Artesian Basin (GAB) of Australia. Total recharge to the system has the potential to occur via three distinct mechanisms namely; diffuse recharge, mountain system recharge, and ephemeral river recharge which all vary both spatially and temporally. Water residence time in thick unsaturated zones can be in the order of 20-30 kyr indicating that the majority of groundwater storage is from diffuse recharge events in the Pleistocene. Mountain system recharge has been previously unrecognised and occurs in isolated regions of elevated Proterozoic outcrop. Of these mechanisms only ephemeral rivers recharge beneath channels that intersect the GAB aquifers is actively occurring under today’s climatic regime, although this component is volumetrically small compared to estimates of groundwater discharge. The system is in a state of hydraulic disequilibrium. Numerical models and the development of analytical solutions to determine the time to reach a new equilibrium indicate that it would take in the order of 50-60 kyr for the system to reach a new steady state. As changes in climate occur at a greater frequency than the time to reach a new steady state it is unlikely the western GAB has ever been in steady state. This has important implications both for how we model and how we manage these largely fossil groundwater resources. We also examined other large arid groundwater basins using the new analytical approach and suggest that many of these large arid basins, where recharge today is close to zero, are not currently in a state of hydraulic equilibrium.