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

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.