A new ecohydraulic management paradigm for salt affected ecosystems and wetlands in low-gradient semi-arid environments

Abstract

Land clearing for dryland agriculture has altered landscape water balance and is associated with severe valley-floor secondary dryland salinity within parts of Australia, South-East Asia, Africa, North and South America. The pervasive hydrologic process-response model is termed the “hillslope recharge-discharge model” (HRD Model), and attributes salinisation to increased hillslope recharge and rising groundwater tables resulting from reduced evapotranspiration potential following land clearing. New research suggests that internal redistribution of surface water from moderately-sloped hillslopes onto low-gradient broad valley floors, termed the “shedding-receiving model”, may be a significant additional salinisation mechanism and alternative management strategies may be required. A meso-scale surface water gauging network and four valley floor surface water-groundwater interaction sites were established in the Toolibin Lake catchment in southwestern Australia. Based on two years of data, we confirm that the shedding-receiving model holds in this landscape, operating in combination with the pervasive HRD model. There is significant rainfall runoff from moderately sloped uplands, onto low gradient valley floors and high transmission losses result from top-down preferential pathway recharge after the break of season. A new management paradigm was proposed to address the internal runoff redistribution and salt exfiltration mechanisms caused by the shedding receiving behaviour. A lowgradient, broad-based channel, approximately 25m wide, 0.4m deep and running at the valley floor gradient (~0.0003 – 0.0015) was designed to carry a 1:3yr flow. Insights gained from this study of the hydrodynamics suggests that this new approach offers significant opportunity to improve local and downstream resource condition