Présentations Colloques

The east coastal plain of South Africa is one of Africa’s most outstanding wetland and coastal sites, and as a result has been given world heritage site status. A series of complex and dynamic wetland, estuary and lake systems are sustained by both groundwater and surface water which are strongly linked in this environment. The increased demand for water from the lakes and their associated groundwater systems has resulted in a significant impact. Moreover, extensive Eucalyptus and pine plantations in the catchment areas transpire large quantities of groundwater, adding extra pressure on the already stressed water resource. This study uses integrated modelling of both the surface water and groundwater resources sustaining the lakes to determine the impacts of current and planned Eucalyptus and pine plantations. The model outcomes were calibrated against groundwater and lake level measurements. This paper will focus on Lake Sibaya which is the largest freshwater lake in South Africa. A series of high dunes separates the lake from the ocean, with a palaeochannel forming the only linkage. The increased demand for water from the lake and associated groundwater system combined with decreasing rainfall has resulted in a significant reduction in lake levels over the past decade. The water balance of Lake Sibaya shows that lake levels fluctuate in response to varying amounts of groundwater and surface water inflow, direct rainfall onto the lake, seepage loss through the coastal dune, abstraction, and evaporation from the lake. Although increases in the rate of water abstraction from the lake together with rapidly increasing pine plantations have had a significant effect on lake levels, it appears that decreasing precipitation is responsible for the recent severe drop in lake levels. Nevertheless, the results indicate that proposed new plantations would impact the already stressed environment significantly.