Impacts of alien plant invasions on water resources and yields from the Western Cape Water Supply System (WCWSS)

Abstract

A key motivation for managing invasive alien plant (IAP) species is their impacts on streamflows, which, for the wetter half of South Africa, are about 970 m³∙ha⁻¹∙a⁻¹ or 1 444 mill. m³∙a⁻¹ (2.9% of naturalised mean annual runoff), comparable to forest plantations. However, the implications of these reductions for the reliability of yields from large water supply systems are less well known. The impacts on yields from the WCWSS were modelled under three invasion scenarios: ‘Baseline’ invasions; increased invasions by 2045 under ‘No management’; and under ‘Effective control’ (i.e. minimal invasions). Monthly streamflow reductions (SFRs) by invasions were simulated using the Pitman rainfall−runoff catchment model, with taxon-specific mean annual and low-flow SFR factors for dryland (upland) invasions and crop factors for riparian invasions. These streamflow reduction sequences were input into the WCWSS yield model and the model was run in stochastic mode for the three scenarios. The 98% assured total system yields were predicted to be ±580 million m³∙a⁻¹ under ‘Effective control’, compared with ±542 million m³∙a⁻¹ under ‘Baseline’ invasions and ±450 mill. m³∙a⁻¹ in 45 years’ time with ‘No management’. The ‘Baseline’ invasions already reduce the yield by 38 mill. m³∙a⁻¹ (two thirds of the capacity of the Wemmershoek Dam) and, in 45 years’ time with no clearing, the reductions would increase to 130 mill. m³∙a⁻¹ (capacity of the Berg River Dam). Therefore IAP-related SFRs can have significant impacts on the yields of large, complex water supply systems. A key reason for this substantial impact on yields is that all the catchments in the WCWSS are invaded, and the invasions are increasing. Invasions also will cost more to clear in the future. So, the best option for all the water-users in the WCWSS is a combined effort to clear the catchments and protect their least expensive source of water.