Modelling and water yield assessment of Lake Sibhayi

Abstract

This study has been undertaken to establish the probable causes of the almost 4 m drop in the level of Lake Sibhayi between 2001 and 2014, to assess the impact of abstractions for domestic water consumption and by commercial plantations on lake levels, and to determine what sustainable yield can be abstracted from Lake Sibhayi. From the analysis and simulations undertaken, it is concluded that the major cause of the drop in the level of Lake Sibhayi was the below-average rainfall over the period 2001 to 2011. However, while the simulation results show that the effect on lake levels of abstractions for domestic usage over this period has been negligible, they do indicate that nearly 1.4 m of the drop in lake level can be attributed to the impact of the afforestation which began in the catchment in the 1990s. A yield analysis of simulated results with historical developments in the catchment for the 65-year period of observed climate record was undertaken using both a fixed minimum allowable lake level or a maximum drop from a reference lake level as criteria for system failure. Results from simulating lake levels using the historical climate record with the area afforested and abstractions levels fixed at 2014 values indicate that no sustainable additional yield is possible because of the sustained decline in both the simulated lake levels and conceptual groundwater store, which would be environmentally, socially and ecologically unacceptable. Preliminary simulated results indicate that the removal of approximately 5 km² of forestry is required to release 1 MCM/yr for domestic abstractions. However, these preliminary results require improved verification of input data and a review of the modelling for increased confidence in the results.