Estimates Of Impacts On Waterflows

David Le Maitre-National Resources and the Environment, CSIR

The new assessment of the impacts of invasive alien plants on the water in South Africa based on the mapping done by Kotze on 2010. The approach took into account basic hydrological principles and factors which limit plant water use and accumulated information on water use by these species in both plantation settings and those typically invaded since the publication of the previous national assessment in 1998.

The taxon with the greatest impacts was the wattles, (Acacia mearnsii) which account for 33.5% of the total reductions followed by Pinus species 18.9% and Eucalyptus 15.1%.

The unit area flow reductions due to pines were 212.1 mm followed by Hakea species (199.5 mm/yr) largely because they occur mainly in high yielding montane fynbos catchments.There are also extensive riparian invasive by eucalyptus, wattles and other species along perennial rivers in semi-arid and arid environment (e.g. Middle and lower
Orange River, low Vaal River) whose impacts have not been included in this assessment.

Prosopis invasions in the Northern Cape result in the further reduction of about 8.94 million a year. There are extensions of Prosopis species in the North West, Free State and Western Cape Provinces which have not been adequately mapped so their impacts cannot be quantified. Wattles (Acacia mearnsii)

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