Satellite-based annual evaporation estimates of invasive alien plant species and native vegetation in South Africa

Wml Meijninger,C Jarmain

doi:10.4314/wsa.v40i1.12

Abstract

In this study we assessed the impact that invasive alien plant species (IAPs), and the clearing thereof by the Working for Water (WFW) programme, have on total evaporation (ET) and the availability of water resources in two highly-invaded provinces of South Africa. The Surface Energy Balance Algorithm for Land (SEBAL) model, using MODIS satellite imagery, was used to estimate the annual total ET at 250 m pixel resolution. ET was estimated for 3 climatically different years for the Western Cape and KwaZulu-Natal. The average annual ET from areas under IAPs, native vegetation, exotic plantation forestry species and control (clearing) areas were compared. The ET of the 5 dominant IAPs (Acacia mearnsii, Acacia saligna, Eucalyptus spp., Hakea spp. and Pinus spp.) in the Western Cape province was 895 mm, which was significantly higher than the ET of most of the native vegetation (thicket 575 mm and fynbos 520 mm), but similar to the ET of dominant exotic plantation forestry species (805 mm). On average, the ET was reduced by 13% to 780 mm, following clearing. In KwaZulu-Natal Province, the ET of the 5 dominant IAPs (Acacia mearnsii, Chromoleana odorata, Eucalyptus spp., Lantana camara and Solanum mauritanium) was 875 mm, which was also higher than the ET of the native vegetation (thicket 755 mm, savanna 685 mm and grassland 640 mm). Following IAP control the ET was decreased by 6%, to 825 mm. This study has demonstrated that spatial ET data with GIS-information on land use can be used to assess the impact of IAPs, and clearing thereof, on water resources. We confirmed results from previous studies, which showed that ET from invaded areas exceeded that from native vegetation. The ET data needs further validation as validation appeared to be impossible. Our results are likely conservative since the majority of invaded areas considered in this analysis represent non-riparian areas. The impact of WFW control of densely-invaded riparian areas is likely more pronounced. We concluded that the clearing of IAPs by the WFW programme has a positive effect on the availability of water resources through a reduction in ET.Keywords: invasive alien plants; indigenous vegetation; remote sensing; water use; evapotranspiration; SEBAL; Western Cape; KwaZulu-Natal

Highlights

Invasive alien plant species (IAPs) are recognised as the ‘most important direct drivers of change in ecosystems’ (Millennium Ecosystem Assessment 2005 p. 14 in Cavaleri and Sack, 2010)
This study has demonstrated that spatial ET data with geographical information system (GIS)-information on land use can be used to assess the impact of invasive alien plant species (IAPs), and clearing thereof, on water resources
In this study we showed that the Surface Energy Balance Algorithm for Land (SEBAL) model using remote-sensing data can be used to determine ET over time and space

Summary

Introduction

Invasive alien plant species (IAPs) are recognised as the ‘most important direct drivers of change in ecosystems’ (Millennium Ecosystem Assessment 2005 p. 14 in Cavaleri and Sack, 2010). Invasive alien plant species (IAPs) are recognised as the ‘most important direct drivers of change in ecosystems’ Researchers believe that IAPs often have a negative impact on water resources through their high water use (ET) compared to native vegetation (Calder and Dye, 2001), little is known about the impact of IAPs on ecological processes (Calder and Dye, 2001; Cavaleri and Sack, 2010). IAPs occur across the SA landscape and it is estimated that about 10 million ha (8.28%) of SA has been invaded to some extent, at an average species density of 17% (Le Maitre et al, 2000). IAPs are controlled through legislation, with a total of 198 IAPs being declared as weeds and an additional species potentially invasive (Henderson, 2001).

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