Soil erosion under different cover management practices during an extreme storm event

During the north-east monsoon period spells of high-intensity rain are common. In the east coast of Peninsular Malaysia in water year 1990-1991, a detailed soil erosion study showed that, collectively, such wet spells were responsible for about 50% of annual soil loss in sloping land. Runoff and soil loss under clean-weeded conditions were compared to those under dense live surface vegetation. The study area supported young cocoa plants shaded by Gliricidia. Several hydrological parameters during the wet spell of November 23 to 26, 1990, with a total rainfall of 464 mm, are discussed. Irrespective of surface cover management, the runoff pattern closely reflected the rainfall pattern throughout the wet spell. But the magnitudes of various hydrological parameters were influenced by cover management, with runoff and rainfall rates reaching their peaks on 25 November 1990. In the later stages of the wet spell, peak runoff rate exceeded peak rainfall rate due to soil saturation and the influence of surface geometry. Although there was continuous rain, eight "bursts" of high rate were recognised. Runoff coefficients gradually increased, reaching 98% for a clean-weeded on 25 November 1990 morning. Soil loss, sediment concentration and runoff rate gradually increased to a peak before tapering off. However, sediment trapped by surface vegetation during smaller storm events were washed out by the first heavy rainstorm, leading to high sediment concentrations on the first day of the wet spell. Although surface vegetation cover limited soil loss and runoff to relatively low levels, a comparison with long-term records showed that during wet spells, their effectiveness was reduced. This wet spell contributed 42 to 52% of total soil loss and 27 to 40% of total runoff, for water year 1990-1991. Areas covered with live surface vegetation recorded thigher percentage contributions to annual soil loss and runoff then clean-weeded areas.