Disconnection of groundwater from surface water causes a fundamental change in hydrology in a forested catchment in south-western Australia

Abstract

In south-western Australia, significant declines in annual rainfall in recent decades have been accompanied by even greater declines in annual streamflow. The disproportionate decline in annual streamflow is perplexing and while there has been speculation about the causes, the mechanisms responsible for the disproportionate decline have not been demonstrated. This study seeks to clarify the role of groundwater in the disproportionate decline in annual streamflow in a small catchment in the jarrah forest by examining records of annual streamflow, stream salinity and groundwater which progressively declined from 1976 to 2011. The records span the transition from connected groundwater–surface water systems to disconnected. This provided a unique opportunity to differentiate the groundwater contribution to streamflow for two reasons. Firstly, because the change in streamflow following disconnection can be largely attributed to streamflow that was previously generated because groundwater was connected. Secondly, because groundwater was the main source of stream salinity and hence stream salinity was a natural tracer which indicated the presence and relative proportion of groundwater in streamflow. Disconnection occurred around 2001 and was signalled by a change in the annual stream salinity signature from moderately high and variable, to low and constant, and by the transition in piezometric levels at the catchment outlet from mostly above ground, to mostly below ground. Following disconnection, the average runoff coefficient which had been slowly declining, abruptly fell by more than half and subsequently remained relatively low and constant. This indicated that whilst groundwater was connected it played a key role in streamflow generation. The contribution by groundwater to streamflow generation was non-linear and was dominant at higher rainfalls. The annual stream salinity signature indicated that direct groundwater discharge to the stream was a relatively minor component, especially at higher rainfalls. Hence connected groundwater contributed to streamflow generation mostly indirectly, thus amplifying other streamflow-generating processes, by facilitating additional surface runoff and/or throughflow which were relatively fresh. As groundwater levels, and hence connectivity, declined, the amplifying effect of groundwater-facilitated streamflow generation also declined. We suggest that the large disparity between the rates of decline in annual rainfall and inflow into reservoirs elsewhere in the forests in south-western Australia mostly reflects the aggregate loss of connectivity in the catchments of streams contributing inflow into the reservoirs.