AbstractsEngineering

St. Lucia is South Africa’s largest and most important estuarine system. The Mfolozi and Mkuze catchments provide the main source of water to the system. Regional climate change may influence future water and sediment yields from the catchments. Other factors include human activities in the form of land use changes, forestation, urbanisation and/or unsustainable agricultural activities leading to land degradation. In this study these changes were evaluated using an analysis of historical rainfall data for the region, and by applying the ACRU model to simulate water and sediment yields incorporating land use changes. Rainfall intensity was a particular focus since it affects the erosion process that underpins sediment yield by providing and maintaining the water-to-sediment ratio within the flow. No consistent evidence of statistically significant changes in mean annual rainfall was found. However, an increase in average intensity of rainfall events across all gauging stations was supported by statistically significant reductions in the number of annual wet-days. An increase in the occurrence of high intensity rainfall (>30mm/day and 50mm/day) was found to be small but statistically significant. Hydrological responses to present land-uses have been evaluated by comparing streamflows and sediment yields generated under natural and current land covers. It was determined that the Mfolozi catchment has undergone a 33% change in land-use from natural conditions. The hydrological impacts of this were a 38% reduction in streamflow accompanied by a 53% increase sediment yield from natural land cover conditions. Subcatchments with high proportions of commercial forest and sugarcane plantations have been identified as the major source of these changes. Using a combination of empirically downscaled rainfall from global climate models, future projections assuming present day land-uses of catchment streamflow and sediment yield have been presented and compared against future projections assuming natural land covers. This was done so as to compare the effects of land-use and climate change on the hydrology of the Mfolozi catchment. Impacts of land-use change were found to be greater than those due to climate change. In a scenario that assumed both occurred, it was found that land-use change was still the dominant driver of hydrological responses, with climate change providing either an amplification or attenuation effect. The findings from this research will provide decision-makers with quantitative guidelines for effective management of the St Lucia Estuary system under different land-use and climatic scenarios.