Impact of invasive alien plants on water provision in selected catchments (original) (raw)
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Invasive alien plants have a negative impact on ecosystem goods and services derived from ecosystems. Consequently, the aggressive spread of invasive alien plants (IAPs) in the river catchments of South Africa is a major threat to, inter alia, water security. The Olifants River catchment is one such a catchment that is under pressure because of the high demand for water from mainly industrial sources and unsustainable land-use, which includes IAPs. This study considered the cost-effectiveness of clearing IAPs and compared these with the cost of a recently constructed dam. The methods used for data collection were semi-structured interviews, site observation, desktop data analysis, and a literature review to assess the impact of IAPs on the catchment's water supply. The outcomes of this study indicate that clearing invasive alien plants is a cost-effective intervention with a Unit Reference Value (URV) of R1.44/m 3 , which compares very favourably with that of the De Hoop dam, the URV for which is R2.93/m 3. These results suggest that clearing invasive alien plants is a cost-effective way of catchment management, as the opportunity cost of not doing so (forfeiting water to the value of R2.93/m 3) is higher than that of protecting the investment in the dam.
Impacts of invading alien plant species on water flows at stand and catchment scales
AoB Plants, 2015
There have been many studies of the diverse impacts of invasions by alien plants but few have assessed impacts on water resources. We reviewed the information on the impacts of invasions on surface runoff and groundwater resources at stand to catchment scales and covering a full annual cycle. Most of the research is South African so the emphasis is on South Africa's major invaders with data from commercial forest plantations where relevant. Catchment studies worldwide have shown that changes in vegetation structure and the physiology of the dominant plant species result in changes in surface runoff and groundwater discharge, whether they involve native or alien plant species. Where there is little change in vegetation structure [e.g. leaf area (index), height, rooting depth and seasonality] the effects of invasions generally are small or undetectable. In South Africa, the most important woody invaders typically are taller and deeper rooted than the native species. The impacts of changes in evaporation (and thus runoff) in dryland settings are constrained by water availability to the plants and, thus, by rainfall. Where the dryland invaders are evergreen and the native vegetation (grass) is seasonal, the increases can reach 300-400 mm/year. Where the native vegetation is evergreen (shrublands) the increases are 200-300 mm/year. Where water availability is greater (riparian settings or shallow water tables), invading tree water-use can reach 1.5-2.0 times that of the same species in a dryland setting. So, riparian invasions have a much greater impact per unit area invaded than dryland invasions. The available data are scattered and incomplete, and there are many gaps and issues that must be addressed before a thorough understanding of the impacts at the site scale can be gained and used in extrapolating to watershed scales, and in converting changes in flows to water supply system yields.
Water SA, 2009
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.
Estimates of the impacts of invasive alien plants on water flows in South Africa
Water SA, 2016
The adverse impacts of alien plant invasions on water flows have been a prime motivation for South Africa's Working for Water Programme. The approach used in this study builds on a previous national assessment in 1998 by incorporating factors that limit plant water-use, information from recent research and improved flow reduction models. The total reduction in flows is estimated to be 1 444 million m 3 •yr −1 or 2.9% of the naturalised mean annual runoff (MAR), less than half of the 3 300 million m 3 •yr −1 estimated in 1998. Two main factors account for this difference: (a) a decrease in the estimated unit-area flow reduction to 970 m 3 •ha −1 •yr −1 compared with 1 900 m 3 •ha −1 •yr −1 estimated in 1998, largely due to the new model being based on more representative reduction factors; and (b) the updated estimate of the condensed invaded area of 1.50 million ha (previously 1.76 million ha), although the taxa mapped for this assessment only accounted for 1.00 million of the 1.76 million ha reported in 1998. Reductions due to invasions in Lesotho are estimated to be about 161 million m 3 •yr −1 and those in Swaziland about 193 million m 3 •yr −1. The taxon with the greatest estimated impact was wattles (Acacia mearnsii, A. dealbata, A. decurrens) with 34.0% of the total reductions, followed by Pinus species (19.3%) and Eucalyptus species (15.8%). The revised estimate is considered on the low side largely because the extent and impacts of riparian invasions have been underestimated. If the current estimates that 4-6% of Acacia mearnsii, Eucalyptus, Populus and Salix invasions are riparian, are adjusted to a more representative 20%, 50%, 80% and 80%, respectively, the total reductions increase by nearly 70% to ~2 444 million m 3 •yr −1. Producing these estimates involved a number of assumptions and extrapolations, and further research is needed to provide more robust estimates of the impacts.
Forest Ecology and Management, 2002
Invasive alien plants are consumptive water-users, and may have reduced river flows in South Africa by about 6.7% according to a broad-scale study. An effective programme to bring the invasions under control would cost about US$ 92 million per year for the next 20 years. This paper reports on studies of four representative catchments (the Sonderend, Keurbooms, Upper Wilge and Sabie-Sand) to assess the impacts and costs of invasions at a scale that is more relevant to managers. Several alien plant species have invaded the catchments. Non-riverine invasions are mainly Pinus and Hakea species in Sonderend and Keurbooms, eucalypts in the Upper Wilge, and pines and scramblers (e.g. Lantana camara) in the Sabie-Sand catchment. Riverine invasions are dominated by Acacia mearnsii and, to a lesser extent, A. dealbata, except in the Sabie-Sand and the lower Sonderend River where Eucalyptus species are important. About 44% of the Sonderend, 54% of the Keurbooms, 2% of the Upper Wilge and 23% of the Sabie-Sand catchments has been invaded to some degree. The corresponding reductions in the natural river flows attributed to these invasions are about 7.2, 22.1, 6.0 and 9.4%. If the invasions are not controlled they could potentially spread, and occupy 51, 77, 70%, respectively, of the first three catchments. At an annual expansion rate of 10-15% this would take about 13, 26 and 63 years, respectively. The invadable areas in the Sabie-Sand catchment are already invaded so invasions will only increase in density. It would take about 26-30 years to reach 100% canopy cover. The projected flow reductions for the four catchments would increase to 41.5, 95.5, 25.1 and 22.3%, respectively. The estimated cost of the control programmes to prevent these losses would be about US$ 13.2, 9.9, 4.1 and 6.6 million for the Sonderend, Keurbooms, Upper Wilge and Sabie-Sand catchments, respectively. Should the catchments be allowed to become fully invaded before control operations were started, then the costs would rise to US$ 86.5, 20.5, 278.0 and 11.1 million, respectively. The impacts and costs are significant and are comparable with those calculated independently for other South African catchments. Water is acknowledged to be a key constraint to economic growth in South Africa and there is considerable pressure for efficient and sustainable use of the limited water resources. The projected impacts would justify control programmes aimed at clearing alien invaders for water conservation. #
Journal of Environmental Protection, 2016
River restoration activities are challenging for the native vegetation and the colonization process. The development of invasive alien plant species (IAS) as well as the development of the pioneer vegetation cover and seed bank were observed in one year prior to construction of a new river bed and within the first two years after construction, along the downstream section of the river Traisen in Austria. One year after completion of the new river channel, the occurrence of IAS above ground decreased while the occurrence of target species increased. Furthermore, no seeds of IAS were detected in the new river bed as a result of proper management of soil movement. Despite the positive impact on the abundance of IAS, the restoration project of the river Traisen shows that the fact that such a restoration activity is a disturbance event must be taken into consideration already during the planning process. Supporting the competitiveness of native plant species is an important tool for reducing the danger of establishment of invasive alien species.
South African Journal of Botany, 2008
Working for Water forms part of the Expanded Public Works Programme of the South African Government, aimed at the sustainable management of natural resources through the control and management of invasive alien plants while enhancing socioeconomic empowerment in South Africa. The programme's name was taken from one of the original motivations: namely, reducing the impacts of invasive alien trees on water resources. A number of studies have looked at the potential impacts of the programme but only one or two have used actual management data to quantify its costs and benefits. This paper is the first, in hopefully a series of papers, on the costs and impacts of the programme over recent years. The paper focuses on the extent, costs and impacts of clearing invasive alien plants from riparian areas. Data were extracted from the Working for Water Information Management System (WIMS) and analysed to assess clearing costs and estimated impacts of clearance on water resources. Some of the most significant findings of the study again illustrate the need to treat invasions as early as possible. Very scattered (1-5%) invasions of selected species for example were between 3 and 25 times cheaper to clear than closed canopy stands (75-100%). On the other hand, unit reference values, used to compare clearing operations in terms of cost efficiency in generating extra water yield, were much higher for low levels of invasion than denser invasions, to the extent that the former's viability could be questioned by the uninformed. However, this was only assessed in terms of extra water generated and not in terms of volumes of water secured, as invasive alien plants spread and become denser if not actively controlled. If left unchecked, water losses increase, which makes the clearing of light infestations much more viable. Overall, it is estimated that around 7% of riparian invasions have been cleared, resulting in significant yield increases. The increased estimated yield of 34.4 million m 3 is about 42% of the yield of the new Berg River Scheme in the Western Cape (81 million m 3). The investment in clearing species known for excessive water use from riparian areas, at a cost of R116 million, was found to be a very good investment. However, it is important to note that the clearing of invasive alien plants will seldom result in the total elimination of shortfalls in water supply and should be seen as part of a package of water resource options to optimize supply, aimed at minimizing wastage of water.
African Journal of Agricultural and Resource Economics, 2017
Environmental degradation caused by invasive alien plants must be remedied in time before the land becomes too heavily degraded for restoration to be successful. This study investigates the cost-benefit analysis of restoring natural capital through clearing invasive alien plants and transforming them into value-added products (VAPs), such as wood chips, timber, firewood, charcoal and briquettes, under three scenarios using a system dynamics modelling approach. The study shows that, if the production of VAPs commenced in 2015, the net present value (NPV) under all scenarios (namely clearing activities paid for by the government alone, clearing activities with 20% co-financed by the private sector, or the do-nothing scenario) resulted in negative values. If, however, the production of VAPs had commenced from the beginning of the model simulation (2008), the cumulative NPV for both the government-funded clearing activities scenario and the 20% private sector co-finance scenario is stro...