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Global freshwater resources are threatened by an ever-growing population and continued economic development, highlighting the need for sustainable water management. Sustainable management must include the control of any additional factors that may aggravate water scarcity, such as invasive alien plants. Water hyacinth ( Eichhornia crassipes ), one of the world’s most destructive invasive plants, presents a direct threat to economically productive water resources. Through high levels of evapotranspiration, water hyacinth leads to substantial water losses that could otherwise be used more productively, thereby creating an externality on water-dependent industries, such as irrigation-fed agriculture. This study provides an economic valuation of the water-saving benefit of water hyacinth control, using Warrenton Weir on the Vaalharts Irrigation Scheme as a case study. A Residual Value Method was employed to estimate the average production value of irrigation water, based on water’s relative proportion of total costs (TC), to serve as a proxy for the value of water lost via evapotranspiration by water hyacinth. Three evapotranspiration to evaporation ratios, derived from the literature, at three levels of invasion (100; 50 and 25% cover), were used to estimate the annual water loss at Warrenton Weir. The average production value of irrigation water was estimated to be R38.71/m 3 , which translated into an annual benefit of between R54 million and R1.18 billion. These results highlight the need for invasive plant control, particularly in economically productive water resources. An alien plant control policy should prioritise invasions of this nature, as they present significant costs to the economy and threaten the sustainability of freshwater resources. Keywords : sustainability, water resources, water hyacinth, water loss, externality, benefit, control

Quantifying the economic water savings benefit of water hyacinth (<i>Eichhornia crassipes</i>) control in the Vaalharts Irrigation Scheme