A strategic study of the impact of invasive alien plants in the high rainfall catchments and riparian zones of South Africa on total surface water yield#

Abstract

The aim of this study was to develop a methodology to determine the impact of upland (non-riparian) invasive alien plants in the high rainfall catchments and riparian areas in all catchments on the total surface water yield available in each of the water management areas of South Africa. This would enable the Department of Water Affairs and Forestry (DWAF) through its public programme Working for Water (WfW) to develop a user charge system for the clearing of invasive alien plants in South Africa. It was found that the total impact of upland invasive alien plants in the high rainfall catchments on the total surface water yield of the country, which included the yield from major dams, minor dams and run-of-river yield, was currently approximately 172 x 10 6 m 3 /a and could go up to as much as 1 410 x 10 6 m 3 /a in the future. The impact varied greatly between water management areas and had the potential to reach 50 % (195 x 10 6 m 3 /a) of registered water use in the Thukela WMA in the future if not controlled. The reduction in yield due to invasive alien plants in the riparian zone in all catchments was estimated to be approximately 523 x 10 6 m 3 /a under current conditions and this could increase to 1 314 x 10 6 m 3 /a if the riparian zone was allowed to become fully invaded. The combined impact was estimated at 4% of current registered water use and could increase to 16 % of registered water use in the future.

Determining a charge for the clearing of invasive alien plant species (IAPs) to augment water supply in South Africa

“…Firstly the shortcomings of the models developed by Cullis et al (2007) need to be taken into account. The study included only those quaternary catchments with a mean annual precipitation (MAP) of more than 800mm.…”

“…It has been indicated (Cullis et al, 2007;Görgens and Van Wilgen, 2004;Van Wilgen et al, 2001) that invasive alien plant species (IAPs) lead to an undesirable reduction of streamflow and water yield. A charge aimed at preventing and eventually controlling invasive alien plant species will therefore render both an ecosystem service (increased water flow to augment the Ecological Reserve) and water as utilisable resources.…”

“…Research has shown that there is an inverse correlation between runoff (or streamflow) and plant biomass loads (Le Maitre et al, 1996;Versfeld et al, 1998), and the link between changes in runoff and the occurrence of invasive alien species has also been shown (Table 1). Cullis et al (2007) The way in which streamflow reduction is influenced by the type and degree of infestation of IAPs, and whether the infestation occurs in mountain catchments or riparian zones, motivated Cullis et al (2007) more or less to the major river basin areas. These estimates are provided in Table 2, (mountain catchment areas), Table 3 (riparian zones) and Table 4 (the total).…”

The impact of water scarcity on economic development initiatives

“…It is also indispensable for economic activities. Water supply in South Africa, however, is limited, unevenly distributed, and negatively impacted by both changes in climate ) and the prevalence and spread of invasive alien plant species (Cullis et al, 2007 andBlignaut et al, 2007). The recognition of the country's precarious water supply conditions has led to innovative initiatives to address it, albeit marginally, such as the water neutral scheme developed and operated by WWF-SA (Nel et al, 2009).…”

Water-use dynamics of an alien-invaded riparian forest within the Mediterranean climate zone of the Western Cape, South Africa

“…At the ecosystem scale, studies indicate that invasive species use 189 % more water than indigenous-dominated stands, particularly in tropical moist forests (Nosetto et al, 2005;Yepez et al, 2005;Fritzsche et al, 2006). In the high rainfall areas of South Africa, invasive alien plants growing in riparian areas are estimated to reduce annual streamflow by 523 × 10 6 m 3 with a predicted annual reduction estimated to be as high as 1314 × 10 6 m 3 if allowed to reach a fully invaded state (Cullis et al, 2007).…”