Preclearing hydrology of the Western Australia wheatbelt: Target for the future?

The wheatbelt of Western Australia largely corresponds to a zone of ancient drainage, characterised by highly variable rainfall, long dry summers, low hydraulic gradients, intermittent surface flows and high regolith salt loads. The accumulation and distribution of salt, the rudimentary aquifers with deep watertables, the intermittent flooding and subsequent transpiration of water from the valley sediments, and the low yields of water reaching the ocean were a product of the underlying physical environment and vegetation types capable of using deeply infiltrated water through the dry season. The hydrological and hydrogeochemical changes induced by widespread clearing of this vegetation for dryland agriculture are profound and enduring. Run-off onto and through the valley floors has increased by a factor of five; combined with local rainfall on these valley floors, the resulting increase in groundwater recharge is filling the deep sedimentary materials and bringing highly saline water to the surface. Diffuse recharge has also increased on the slopes and ridges, with saline watertables rising in these lateritic formations as well, providing additional hydraulic heads forcing groundwater towards the valleys. The resulting increase in the groundwater discharge areas is projected to greatly increase flooding risk downstream into the future. A variety of natural, built and agricultural assets are either already impacted or at risk to these phenomena. It is hypothesised that restoring the original hydraulic and hydrological functions of the system will lead to its recovery. This raises several issues: can we design remedies in terms of restoring the original rates of flux ( recharge, runoff, etc) or in terms of the original balances ( recharge less than aquifer discharge, input of salt into the root zone equal to output)? Secondly, to what degree can revegetation or engineering now restore these original conditions? Finally, we examine the potential for the landscape to recover to its original hydrological and hydrogeochemical state once salinised. Given the advanced state of saline watertable development, with its implications for successful revegetation and restoration of valley transpiration, the changes in soil structure and chemistry, and the immediate implications to valued assets, we posit that an aim of restoring the landscape solely with revegetation, either in terms of rates or balances, is not feasible or even possible. To a degree, one can only restore certain aspects of the original balance via revegetation combined with discharge enhancement and flood mitigation.