Fire can explain rainfall-runoff shifts during and after drought

State shifts in rainfall-runoff relationships, driven by disturbances such as droughts and bushfires, can persist long after the disturbance, resulting in reduced streamflow per unit rainfall. However, droughts and fires are highly correlated, and fires can also affect runoff patterns. In this study, we investigated the joint influence of fires and prolonged drought on rainfall-runoff relationships in the Australian Capital Territory (ACT) and neighbouring Queanbeyan catchments. State shifts in rainfall-runoff relationships were detected using Hidden Markov Modelling (HMM) of runoff, incorporating Kuczera fire parameters that characterise the duration and magnitude of streamflow reduction following fire. Hidden Markov Models avoid the need for a priori definition of drought periods and provide a flexible framework to identify the state changes in the rainfall-runoff relationship. We found that: (1) Fire can explain apparent state shifts in rainfall-runoff relationships during and after drought. Based on previous literature, we hypothesised the existence of multiple hydrological states in the western forested region, however the inclusion of fire as a factor removed most of the two-state behaviour (3 of 6 gauges); (2) State shifts, in this study, were more common in non-urban non-forested catchments (4 gauges) rather than the forested ones (1 of 7 gauges), contrary to expectation; (3) Moreover, almost all gauges (13 of 16) did not display a long-term rainfall-runoff state shift following drought, suggesting that the study catchments are resilient to drought. The catchments exhibited recovery behaviour even when fire events that occurred amid prolonged drought were considered for the analysis. These findings underscore the importance of integrating both fire and drought in hydrological modelling to better understand their combined effects on catchment behaviour.