Field estimates of groundwater discharge – Great Artesian Basin, South Australia

Abstract

Understanding the water balance of large groundwater systems is fundamental for the sustainable management of the resource. However, measuring many of the major components of the water balance pose considerable challenges and are thus estimated through modelling. A shortcoming of the modelling approach is that the water balance may have high bias and uncertainty due to insufficient information. 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. This paper reports on the use of field measurements and remote sensing to estimate the evaporative discharge along the southwestern margin of the GAB in South Australia, as this flux provides an upper bounds of vertical leakage along this part of the GAB margin. These discharge estimates are compared to published estimates of vertical leakage from modelled simulations by the Bureau of Rural Sciences (BRS) steady-state GABFLOW model. Field estimates of evaporative discharge were made using eddy covariance station and micro-lysimeter measurements, and inversion of chloride/isotope soil profile measurments. The field estimates were assigned to three major land-types associated with areas of increasingly higher evaporative discharge and progressively decreasing depths to the water table. These land-types 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 evaporative discharge (0.5-10, 10-100 and 100-300 mm y -1 ) were assigned to the major land-types; land- types were mapped using both quantitative classification of remote sensing imagery and semi-quantitative mapping of landforms using a variety of remotely sensed and field data. The remote sensing method provided a probable minimum estimate, due to the spatial resolution of the satellite data being unable to map areas with heterogeneous mixtures of discharge and non-discharge surface characteristics. In contrast, the digital elevation method provided a maximum area of higher evaporative discharge (>10 mm y -1 ) due to its more interpretative nature that lumped areas together, and the fact that fine-scale microtopography and vegetation variations within the mapped areas are largely ignored. The evaporative discharge areas mapped by this project were separated into western and eastern sub-basins, reflecting groundwater inflow from the western margin and eastern margin of the GAB respectively, and a mixing zone that received inflow from both sub- basins. The higher evaporative discharge zones mapped by quantitative classification of satellite data are 8-28% of the total South Australian vertical leakage component modelled by BRS (Table 4). Areas of evaporative discharge fed only from groundwater flowing from the western sub-basin are 7-24%, while areas from the mixing zone are 1-3%, and areas from the eastern sub-basin account for <1% of the modelled total vertical leakage for South Australia, respectively. In comparison, the higher evaporative discharge zones estimated by landform mapping are 73-251% of the total vertical leakage component modelled by BRS (Table 4), with the western sub-basin being 64-216%, the mixing zone being 5-22% and the eastern sub-basin being 4-13% of the modelled total vertical leakage, respectively. 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 evaporative discharge in the high discharge zones located around the Basin margins, implying that vertical leakage rates distal to the margins are very low, and/or the inflow to this part of the GAB is currently underestimated. 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 east are likely to be more complex than for the western sub-basin.