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Groundwater use by alien invasive plants: assessing the impact of Prosopis spp invasions on water supply to groundwater dependent communities
Expanded Title:Invasive alien plants are estimated to have invaded at least 10 million ha of South Africa to some degree and are spreading at about 5% per annum. The deep rooted desert adapted shrub or tree, Prosopis spp, is a major invader species in the arid and semi-arid parts of the country. Near the turn of the 19th century, six Prosopis species from Central America were introduced to Namibia and the arid parts of South Africa for fodder, fuel and shade. Here they have hybridised and spread rapidly. Extensive stands of this woody weed currently grow in many parts of South Africa including the North West, Northern Cape, Free State, Western Cape and parts of the Eastern Cape Provinces. The International Union for the Conservation of Nature (IUCN) declared Prosopis to be one of the world’s worst invasive species (Bromilow, 2010). Recent studies show that the average annual rate of spread of Prosopis in the Northern Cape, for example, is around 15% in upland areas and up to 30% in riparian areas (van den Berg, 2010). The species occupied only about 128 000 ha in 1974 in the Province. It then increased rapidly to over 1.5 million ha (> 4% of the Province) by 2007, roughly doubling between 2004 and 2007. Given the ability of Prosopis to use groundwater, its rapid spread poses a serious threat on water supply to groundwater dependent communities especially in the drier parts of the country. So this study seeks to quantify the impact of Prosopis invasions on groundwater in parts of South Africa where communities rely on groundwater. This study demonstrated that deep rooted indigenous trees growing in arid to semi-arid environments are equally likely to use as much or more water per plant than invasive alien plants. However, the pronounced impacts of Prosopis invasions on groundwater are a result of the ability of the species to form large plant densities as opposed to higher water use rates by individual trees per se. As a consequence programs to remove Prosopis so as to recover groundwater should target areas were the densities are likely to be high. This study suggests that clearing one hectare of Prosopis at this specific site would release significant volumes of groundwater (~ 4 200 m3/ha/yr). This quantity of water is sufficient to meet the irrigation water requirements of one hectare of small crops e.g. cereals or pastures that are grown in the study area. Removal of Prosopis led to an immediate reduction in the rate of decline of groundwater levels, but the water levels continued to drop due to up take by the deep rooted indigenous vegetation. Clearing of Prosopis is therefore likely an effective strategy to increase the available groundwater volumes in groundwater dependent communities. Lastly, a model that integrates remote sensing and in situ inputs has shown great promise in quantifying the water requirements of invasive alien plants in and arid to semi-arid catchment.
Date Published:01/05/2018
Document Type:Research Report
Document Subjects:Ecosystem - Alien Species
Document Keywords:Ground Water
Document Format:Report
Document File Type:pdf
Research Report Type:Standard
WRC Report No:2256/1/18
ISBN No:978-1-4312-0974-3
Authors:Dzikiti S; Bugan R; Le Maitre D; Ntshidi Z; Ramoelo A; Gush M; Jovanovic N; Schachtschneider K
Project No:K5/2256
Originator:WRC
Organizations:CSIR; University of Cape Town; WWF SA
Document Size:6 768 KB
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