Conclusions

A few previous studies using statistical approaches have reported that multi-year drought can induce a shift in the catchment rainfall-runoff relationship, but rarely have studies provided process-level interpretation of such shifts. Consistent with the results of previous research, the current study demonstrated that a change in the rainfall-runoff relationship was detected after the beginning of a prolonged drought period (1997) in the Wee Jasper catchment in New South Wales, Australia. However, the new analysis approach proposed in this study found that the change in the rainfall-runoff relationship is induced by an increase of catchment active storage capacity and a decrease in soil moisture resulting from persistent evapotranspiration of deep-rooted woody vegetation during the drought, leading to a decline in groundwater level and deep soil moisture.
This study concluded that the combination of data assimilation and a hydrological model was a suitable approach for detecting the hydrological non-stationarity caused by prolonged drought. This approach not only can detect changes in rainfall-runoff relationships, but also can identify the driving factors for such changes at the process-level. The method used in this study can provide assistance in developing strategies and management practices to mitigate the negative effects of prolonged drought, and in developing preparedness and adaptation strategies for the challenges of climate change which will likely increase the frequency and severity of drought in the future.