SIMULATOR EXPERIMENTS OF THE VARIED CONSEQUENCES OF RAIN-FOREST LOGGING FOR RUNOFF AND EROSION

This paper investigates the hydrological and erosional consequences of logging using small hillslope plots from Sabah, Malaysia, subjected to simulated rainfall, and laboratory measurements of the effects of compaction on soil permeability. Following logging there is a four-fold increase in runoff volumes resulting from a decrease in saturated permeability from 1 x 10(-5) m/s in undisturbed rain forest to 1 x 10(-8) m/s in recently logged parts of the catchment. Following logging sediment yield increases by over two orders of magnitude. These results help to elucidate both the average changes in catchment response due to rain forest logging, and the likely spatial variability that results from different degrees of landuse change in different parts of the catchment. Catchments in the Humid Tropics frequently consist of spatial units at different stages of regeneration. This paper also considers the effects of vegetation regeneration by investigating plots-from areas logged in 1988-89 and in 1991, to help to evaluate the significance of spatial variability within catchments that have been logged at different times in the past. The results indicate that runoff volumes de dine considerably over a four-year regeneration period, although they become more spatially variable and can remain high in some areas. Sediment yields decline systematically with the length of the regeneration period. After four years sediment yields are close to those of undisturbed rain forest.