The Northern Australia Water Resource Assessment (to conclude in June 2018) will provide a range of information to facilitate economic development in three regions located across northern Australia. Such information includes evaluations of the scale and opportunities for groundwater resource development whilst recognising the potential environmental, social, cultural and economic benefits, impacts and risks of increased future usage. In areas where groundwater baseflow sustains groundwater-dependent ecosystems, a key limitation on the potential for groundwater extraction is the risk of streamflow depletion. This is defined as the fraction of extracted water that would otherwise discharge to a stream at a given time after the onset of (typically continuous) groundwater extraction.
The potential for streamflow depletion by groundwater extraction has historically been estimated using analytical solutions and/or using deterministic numerical models. In the present study, numerical solutions were incorporated in a Monte Carlo framework in order to account for both conceptual and parametric sources of uncertainty. Sources of conceptual uncertainty assessed included variations in hydrostratigraphy, including the presence of a surficial aquitard or multiple aquifers, as well as variations in bore penetration depth and screen extent. Sources of parametric uncertainty assessed included hydrogeological properties, streambed conductance, and rates of extraction. Numerical solutions enabled testing each of these assumptions, as well as providing the ability to simulate discontinuous extraction regimes. Furthermore, numerical solutions also provided the ability to predict cumulative streamflow depletion over the duration of extraction.
Global sensitivity analysis metrics and simple scatterplot analyses were used to identify model parameters to which predictions of streamflow depletion were most sensitive. This research serves as a practical demonstration of best-practice methods for the estimation of streamflow depletion uncertainty. The methodology will support transparent, comprehensive and robust groundwater allocation policy where groundwater extraction occurs (or is proposed to occur) in the vicinity of groundwater-fed streams.