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Abstract
Floodplain wetlands are globally biodiverse and productive ecosystems, offering a functional mosaic of habitats. Inundation regimes change the availability of these habitats through time, depending on flooding and drying patterns. Many are altered by water resource developments, requiring restoration using managed environmental flow flooding which is not well supported by good understanding. Comprehension of inundation regimes over large spatial and long temporal scales is challenging but important for informing effective management. I investigated the spatio-temporal variability of inundation regimes, as a driver of floodplain heterogeneity of the Macquarie Marshes, a Ramsar-listed wetland, reliant on its flow regime for flooding. This large (200,000 ha) floodplain wetland in the Murray-Darling Basin of south-eastern Australia, is supplied by the Macquarie River, regulated by large dams and diversions upstream. Environmental water (~350,000 ML when dams are full) is targeted to floodplain inundation and consequent environmental outcomes. I examined variation in inundation extent and its relationship with flows, inundation regime change, and inundation regime components for persistence of flood-dependent vegetation.
I classified inundation extent from the Landsat satellite archive with high accuracy, across a variable landscape and over long time periods. My spatial analyses, through varying temporal windows, revealed high spatial and temporal variability of inundation regimes. Drying across the inundation frequency gradient equated to decadal loss of inundated area, a function of lagged antecedent measures of rivers flows. Variability of all inundation regime components: frequency, duration and time-since-last-flood measured over a 20-year temporal window explained broad vegetation group distribution across the inundation gradient, providing insights into flood dependency (e.g. non-woody wetland vegetation flooded every 1 to 1.4 years, with a long-term average duration of 41-96 days, a maximum duration averaging 7.5-11.2 months and a time-since-last-flood of 0.16-2.3 years).
In conclusion, variable inundation regimes were highly reliant on river flows and vital for maintaining a heterogeneous mosaic of floodplain wetland vegetation. Measuring inundation regime dynamics using remote sensing is critical for environmental water management, aiming to maintain and restore floodplain wetland vegetation by using river flows to inundate large targeted areas.