From drainage to recharge to discharge: Some timelags in subsurface hydrology

In arid areas, low rates of groundwater recharge and thick unsaturated zones mean that significant time lags can occur between changes in drainage below plant root zones and changes in aquifer recharge. The magnitude of the timelag can be approximated using simple analytical expressions, or more accurately determined using numerical models of soil water movement. It is related to the thickness of the unsaturated zone, the magnitude of the drainage rates, and the soil hydraulic properties. In southern Australia, in areas where water tables are 30 m below the land surface, timelags of 30-200 years occur between clearing of native vegetation for agriculture and an increase in groundwater recharge. It is predicted that similar timelags would occur between revegetation and a reduction in recharge. The timelag between a change in groundwater recharge and the change in groundwater discharge is related to the distance between the recharge and discharge areas, and the aquifer transmissivity and specific yield. For large groundwater basins with low recharge fluxes, this timelag can be many hundreds or thousands of years. In some cases, this allows groundwater extraction at rates well in excess of recharge rates to continue for a number of years before the impact of this policy will be seen. On the other hand, in southern Australia revegetation to reduce recharge and hence lower watertables and reduce saline groundwater flows to one of Australia's most important rivers (the Murray River) are being considered. The timelag between the reduction in recharge and the consequent reduction in discharge limits the effectiveness of this management option in areas further than a few kilometre from the river.