Impact of change in vegetation cover on groundwater recharge to a phreatic aquifer in Western Australia: assessment of several recharge estimation techniques

Groundwater levels in a deep phreatic aquifer in the northern Perth Basin, Western Australia have risen between 10 and 55 cm/y over the last three decades due to the replacement of deep-rooted native vegetation with pasture and annual crops. This has sparked debate over increasing groundwater allocation limits to meet growing demand. Estimates of recent and historical rates of groundwater recharge are needed to manage the resource effectively. The Parmelia Formation, composed predominantly of sandstone, supplies domestic, stock and production bores for rural towns. Groundwater recharge was estimated using several recharge estimation techniques including the water-table fluctuation method and tracer techniques; groundwater chloride, chlorofluorocarbons (CFCs) and C-14. Vertical fluxes of soilwater under pasture and native vegetation were determined using chloride as a tracer. Apparent groundwater ages determined using CFCs and carbon-14 are generally younger than 40 years in piezometers that are screened within a few metres of the water-table from recharge under pasture, while groundwater is older than several thousand years in bores that are screened well below the water-table, and mixed waters occur at discharge areas. Recharge-rate estimates obtained under native vegetation are mainly <12 mm/y, whereas after replacement with pasture and annual crops, recharge rates are generally 20 – 50 mm/y. The presence of thick clay lenses in some areas may be responsible for observed delays of approximately one decade in the water-table response to land-use change. The study highlighted the fact that several recharge estimation techniques are required for better understanding of recharge processes and evaluation of recharge.