VEGETATION MODIFICATION OF RAINFALL CHARACTERISTICS: IMPLICATIONS FOR RAINFALL EROSIVITY FOLLOWING LOGGING IN SABAH, MALAYSIA

Abstract

Modelling soil erosion following forest disturbance requires new data on a variety of parameters which control runoff generation and soil detachment. In the study area, Sabah, East Malaysia, there are no data on rainfall transformation by canopies particularly changing drop size distributions and rainfall kinetic energy at different intensities. Filter paper staining techniques were used to measure rain drop size distributions and calculate rainfall intensity and total kinetic energy for rainstorms under open sky and closed canopy conditions in an area of lowland dipterocarp forest. The results show a positive correlation between median drop size, rainfall intensity and kinetic energy for open sky situations, consistent with studies from elsewhere. Mean drop size for all samples is around 2.2mm. For closed canopy situations there is no such relationship in rainfall parameters, all drops being around 5.5 mm in diameter regardless of rainfall intensity. With interception losses of 29 % in the study area, the modification to total kinetic energy following forest disturbance can be assessed. The results show that above rainfall intensities of 20 mm h' open sky rainfall is more erosive than that falling through the canopy. Since most rainstorms are above 20mm h"1, canopy disturbance is likely to increase erosion rates, and needs to be included when modelling soil erosion. These results provide essential inputs to new physically-based models being applied for the first time to the issue of rain forest logging effects on soil erosion in Sabah.