The sustainable development of water resources: History, financial costs, and benefits of alien plant control programmes
The water resources of the Western Cape province, and the catchment areas that produce them, need to be carefully managed if future demands for water are to be met. Good catchment management, in the form of programmes to control alien invasive plants, provides an additional means for preventing losses of water, but needs to be justified financially. Previous programmes aimed at the control of alien plants have been curtailed due to a lack of funds in the recent past, as they were conducted for reasons of nature, rather than water, conservation. However, recent studies have highlighted the impacts of alien plants on water resources, and have forced planners to take these impacts into account. In this paper, we review the history of alien plant control programmes, and assess the costs (financial) and benefits (in the form of reduced losses of water from invaded catchments) that would arise from such control. We use the standard practice of discounting future costs and benefits arising from planned and existing water supply schemes to estimate the relative efficiency of the schemes. By modelling the spread and effects of alien plants on streamflow, we show that alien plant control is effective and efficient. In the case of the proposed Skuifraam scheme (the preferred next option to supply water to Cape Town), water can be delivered at a cost of 57 and 59 c kl-1, respectively, with and without the management of alien plants, for example, indicating that such management is cost-effective. Clearing invasive plants from the existing Theewaterskloof catchment would deliver additional water at only 13.6% of the cost of delivery from the new Skuifraam scheme (unit reference values of 8 and 59 c kl-1 respectively). The analyses also show that an early investment in clearing is more cost effective than a later investment, as costs increase and yields decrease the longer the catchment is left to become more invaded. Many other benefits, including avoiding serious impacts on biodiversity, catchment stability and fire management, and enhancing social upliftment, have not been included in this assessment, and add to the argument for implementation of the programme for clearing invasive alien plants.