This study quantified the water savings from controlling the invasive water hyacinth plant in the Vaalharts Irrigation Scheme in South Africa. Water hyacinth leads to substantial water losses through high evapotranspiration rates, reducing the available water for productive irrigation agriculture. The study estimated the annual water loss from three levels of water hyacinth coverage at a weir using evapotranspiration ratios. This lost water was valued between R9 billion to R500 million based on the average production value of irrigation water. The results highlight the need to control invasive plants that threaten economically important water resources and sustainability.

1. Quantifying the water savings benefit of water hyacinth (Eichhornia
crassipes) control in the Vaalharts Irrigation Scheme
Reinhardt Arp
Abstract
Global fresh water resources are threatened by an ever growing population and continued
economic development, highlighting the need for sustainable water management. Sustainable
management must also control any additional factors that may aggravate water scarcity.
Invasive alien plants present such a threat, and pose a particular problem for water scarce
countries. South Africa is not immune to this global phenomenon, with plant invasions
estimated to carry an annual loss of R5.8 billion in water provisioning services. Water
hyacinth (Eichhornia crassipes) presents a direct threat to economically productive water
resources, such as irrigation water. Through high levels of evapotranspiration, water hyacinth
leads to substantial water losses that could otherwise be used more productively, thereby
creating an externality on irrigation fed agriculture. This study provides an economic
valuation of the benefits 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, to serve as a proxy for the value of water lost
via evapotranspiration by water hyacinth. Three evapotranspiration to evaporation ratios 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
R300 / m3, which translated into an annual benefit of between R9 billion and R500 million.
The results of this research highlight the need for invasive plant control, particularly in
economically productive water resources. Alien plant control policy should prioritise
invasions of this nature, as they present significant costs to the economy and threaten the
sustainability of fresh water resources.
Keywords: sustainability, water resources, water hyacinth, benefit of control