Do catchment state shifts have multiple thresholds and thus exhibit finite resilience?

Partitioning annual and seasonal rainfall to runoff has been shown to shift and often not recover from droughts. This non-recovery suggests that a catchment can have different long-term runoff rates for the same rainfall; that is, multiple stable states. Here, we aim to statistically examine long-term streamflow observations to assess if multiple thresholds between states exist. If multiple thresholds are found then this provides strong evidence for catchments having a finite resilience to drought. We evaluated the contribution of climatic drivers (rainfall and PET) to Hidden Markov transition probabilities using a historical wetness forcing (HWF). The HWF was then input into different logistic functions to derive the transition probabilities. One function was a standard logistic, demonstrating a single-threshold for drought and recovery. A second had two logistic displacement functions, identifying one threshold for drought and another threshold for recovery. We identified the number of thresholds by developing non-homogenous Hidden Markov Models for annual streamflow using rainfall as a co-variate; that is, the state transition probabilities changed with the catchment wetness. We applied 32 variants of the model to 158 unregulated catchments in Victoria, Australia (Fig. 1). The most parsimonious models were then identified using the Akaike Information Criterion. Results show that 75% of catchments (n=118) exhibit two runoff states. Of these 118, 80 catchments are found to have transition probabilities varying with the climatic drivers. Of these 80, 17 have a single threshold. Conversely, 63 catchments have two thresholds. This suggests that 40% of the study catchments show evidence of one climate threshold for entering into a low runoff state and a second and higher threshold for recovering back to a normal runoff state. Overall, the results provide further evidence that catchments can have multiple coexistent stable states and a finite resilience to drought.