Framework for incorporating spectral observations into steady state analysis of groundwater discharge around the margins of the Great Artesian Basin

Diffuse groundwater discharge (also known as vertical leakage) forms an important and poorly constrained component of the water balance of the Great Artesian Basin (GAB), Australia’s largest groundwater resource. Around the south-western margin of the GAB, the diffuse leakage from the artesian aquifer discharges into the unconfined water table and subsequently into the atmosphere via evapotranspiration (ET). Under steady state conditions (i.e. prior to the development of the GAB), the evaporative flux, in addition to natural artesian discharge from springs, are considered to equal the potentiometric head driven flow to the Basin margin. Since 2007, a project has been collecting field data on the evaporative discharge rates using a number of essentially point-based techniques. These techniques involved bare soil evaporation measured by microlysimeters, evapotranspiration measured by an eddy covariance flux station, and long-term evaporative discharge rates through the soil column estimated from soil solute profiles using an advection-diffusion model. Spatial heterogeneity provides a major challenge in utilizing these data for estimating the diffuse groundwater discharge over a large area. In this study we use spectral data from the ASTER, Landsat and Quickbird satellites, in addition to geomorphic mapping and digital elevation data, to characterize discharge zones according to a conceptual framework based on the physical attributes of varying evaporative discharge, in particular the salt content at the surface, soil moisture and thermal properties. The conceptual framework aims to relate subsurface processes, and ultimately rates of diffuse discharge, with their expected surface expressions. The framework classifies discharge areas into zones which represent approximately order of magnitude variations in discharge rates, as defined by their subsurface processes. The field-based estimates of discharge ranges are assigned to the land surface classes according to spatial location and spectral similarities. This method was applied to a study area (Public House Springs) located on the margin of the GAB with the Gammon Ranges in South Australia. An analysis of groundwater flux rates over the study area was conducted to determine if the estimated evaporative discharge and artesian spring discharge rates complied with steady state conditions. Inflow to the model domain was estimated with Darcy’s Equation, using measured differences between the artesian head and the artesian spring mean elevation and best estimates of aquifer parameters from other studies of the GAB. This approach provides a framework for both estimating diffuse discharge at large spatial scales and further investigating the effects of uncertainties in the major input data sources.