Identification of the major hydrological threats for two clay pan wetlands in the South West of Australia

Abstract

This study presents the findings of a wetland ecohydrological model (WET-0D), used to recreate a historical water regime and predict the future water regime for two clay pan wetlands in South West Western Australia. WET-0D simulates the major hydrological fluxes through three conceptual water storages including the open water/lake, and surrounding unsaturated and saturated zones. Groundwater - soil water balance - vegetation (GSV) dynamics are modelled with plant biomass simulated as three functional vegetation groups with differing water uptake strategies and dependence on water availability. The wetland model was driven by a simple catchment water balance model, using historical climate data from the Bureau of Meteorology (BoM). To simulate the potential impact of climate change on wetland ecohydrology, statistically downscaled output from a Global Climate Model (GCM), based on the IPCC SRES A2 scenario, was used to drive the models to predict potential future water regimes. This allowed us to gain an insight of the impact of projected drying climate on the clay pan ecosystems. Although both clay pan catchments experience very similar climates, differences in the partitioning of rainfall and subsequent flow generation, due to different vegetation, soil type and topography, results in dissimilar hydrological regimes. Differences in the hydrological regimes alter the way predicted climate change affects water flux and hydroperiod (period of surface flooding of a wetland) in both clay pan systems. Historically, the modelling predicts the lake level in the North East clay pan is more dependent on overland flow, while the South West clay pan is more dependent on shallow groundwater flow from a seasonal aquifer. Under a drying climate the modelling predicts, the South West clay pan will become increasingly overland flow dependent. However, the shallow groundwater inputs to the clay pan prolong inundation by reducing the rate of seepage from the clay pan. The partial clearing of the catchment area for the South West clay pan has maximised groundwater recharge efficiency allowing maintenance of ecological water requirements under a drying climate. The North East clay pan is under greater threat due to the reliance of surface water inflow and the lack of groundwater input due to differences in catchment characteristics.