Evaluating the cumulative rainfall deviation approach for projecting groundwater levels under future climate

In south-western Australia (SWA) groundwater levels have been declining under the changing climate associated with a decline of rainfall. Possible future groundwater yields in SWA have been estimated under a range of climate change scenarios using a number of numerical groundwater models. For the Northern Perth Basin (NPB) in SWA, where no groundwater models were available, a relatively simple statistical method CDFM (Cumulative Deviation from Mean) has been applied using HARTT (Hydrograph Analysis and Rainfall Time Trends), an automated derivation of the CDFM. This study has evaluated the potential of the CDFM to project groundwater levels under various future climate scenarios in the NPB. Firstly, HARTT projections were validated by comparing with the modelled hydrographs in areas where numerical groundwater models were available. It was evident that HARTT may overestimate future declines or rises in groundwater levels depending on the time a new climate regime is imposed on the model. Secondly, HARTT was applied to suitable bores in the NPB under future climate scenarios. HARTT projected a slight decline under a drier future climate than under the historical future climate and a moderate or slight rise in groundwater levels under a wetter future climate. If historical climatic conditions continue until 2030, groundwater levels are expected to slightly rise in the NPB.