Long-term ecohydrological response of wetlands in managed catchments to climate variability

Predicting the response of wetlands in regulated catchments is challenging because their natural flow regime has been replaced by managed flows that depend on environmental allocations, as well as urban and agricultural demands. Many of such wetlands in semi-arid Australia have been significantly impacted by the combination of increased human demands and climate conditions characterized by pronounced inter- and intra-annual variability. For instance, the Macquarie Marshes in New South Wales have seen reductions in flow magnitude and variability due to human activities, leading to a 40–50% decrease in wetland area despite the provision of environmental flow allocations. Predicting ecological responses under altered flow regimes and variable climate remains a complex undertaking, as historical records, while informative, capture only a limited range of climate variability.

 

We employ stochastic climate data to investigate the long-term response of the Macquarie Marshes to climate variability. Two contrasting scenarios are assessed: one representing natural conditions (best-case) and another reflecting current managed conditions with complete removal of environmental flow allocations (worst-case). These scenarios are simulated using a catchment model of the Macquarie Valley (developed with WATHNET5) coupled with an ecological response model of the marshes, which predicts vegetation health (woody and non-woody) and bird breeding suitability (Ibis and Egret species). Each scenario is run across 100 replicates of 110-year sequences of stochastically generated climate data derived from historical records.

 

Compared to the best-case scenario, the worst-case scenario leads to an average 30% reduction in non-woody vegetation cover, with extended periods of severely diminished coverage. Woodland and river red gum forests experience a 30% decrease in the time they remain in good condition, coupled with up to a 10% increase in time spent in poor condition. These vegetation changes significantly impact bird breeding opportunities, reducing the number of favourable events by approximately 50%.