Determining vertical leakage from the Great Artesian Basin, Australia, through up-scaling field estimates of phreatic evapotranspiration

Abstract

Summary Understanding the water balance of large groundwater systems is fundamental for the sustainable management of the resource. The vertical leakage (i.e. discharge to upper aquifers or the unconfined water table) component of the Great Artesian Basin (GAB) is an example of a poorly constrained but large component of the water balance of Australia’s largest groundwater resource. Field estimates of phreatic evapotranspiration (ET) were made at discharge zones along the southwestern margin of the GAB using eddy covariance station and micro-lysimeter measurements, and inversion of chloride/isotope soil profile measurements. The field estimates were assigned to three major landforms associated with areas of increasingly higher evaporative discharge and progressively decreasing depths to the water table. These landforms were mapped using remote sensing and digital elevation data, with characteristically higher soil moisture, salt precipitation, and lower surface temperature compared to areas distal to discharge zones. Based on the field measurements, broad ranges of phreatic ET (0.5–10, 10–100 and 100–300 mm y −1 ) were assigned to the major land-types. The higher phreatic ET discharge zones mapped by supervised classification of satellite data are 8–28% of the total regional vertical leakage component estimated by numerical modelling of the GAB. In comparison, the higher discharge zones estimated by landform mapping are 73–251% of the total vertical leakage component estimated by modelling. The mapped distribution of the high discharge areas has important implications for modelling of the GAB. In the western sub-basin, most of the estimated recharge can be accounted for by phreatic ET in the high discharge zones located around the Basin margins, implying that vertical leakage rates distal to the margins are very low and that discharge may exceed current recharge. In contrast, the results for the eastern sub-basin suggest that vertical leakage rates around the South Australian portion of the Basin margin are low and that more of the vertical leakage component in the eastern sub-basin is occurring distal to the Basin margins. Consequently, the pathways for vertical leakage in the eastern sub-basin are likely to be more complex than for the western sub-basin.