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ABSTRACT
The mountains of the Western Cape Province fulfil a vital water supply function in the region. For the protection of this water supply, it is important to understand natural soil erosion rates in these mountains and human influence on these. The main land management practices are prescribed burning of natural vegetation for conservation of bio-diversity and water yield, and the use of mountain land for the production of timber from plantations of exotic tree species. Wildfires are an inherent risk during the dry summers. This paper reports various studies at different scales a over 20 year period with most estimates of erosion being indirect, through measurement of sediment in streams.
Small increases in sediment yields have been measured following prescribed burning of catchments which are covered in fynbos, the natural sclerophyllous scrub vegetation of the region. The bulk of sediment is carried in stormflows, and individual storms may dominate the annual sediment yield. Soil losses per unit area off small mid-slope plots are much higher than those measured at the catchment level, and estimated erosion rates are inversely proportional to the size of the erosion plot. Riparian zones appear to be important in keeping sediment delivery ratios low.
High intensity wildfires in the late dry season have the potential to cause large increases in stormflow and sediment yields in burned catchments. This is particularly the case in timber plantations where fuel loads are higher and may be concentrated in slash piles, and where road systems may aid the transport of surface runoff. Marked hydrological responses to wildfire have been associated with fire-induced water repellency in the soils of the burned catchments.