RUNOFF GENERATION UNDER DIFFERENT LAND USES AT HORTON PLAINS

Abstract

Surface runoff occurs when the rate of precipitation exceeds' the rate ofinfiltration. Under different land-use types the soil properties that can affect theway in which runofftakes place may va~v significantly. At Horton Plains, thereare two distinct vegetation types, cloudforest and grassland (natural as well asre-established after cultivation about twenty years ago), which might besupposed to affect runoff. This study was conducted to explain the runoffgeneration process at these sites by determining soil texture, organic mailercontent, bulk density, infiltration and hydraulic conductivity under each landuse. The bulk density of the top 100 em of the soil is exceptionally low in allland-use types, because of the high organic matter content. This low densityincreases the infiltration rate. Comparison of' rainfall intensities with infiltrationrates indicated that in forest and natural grassland, for all the rainfall events ofthe first six months of' 1996, rainwater would have entered the soil withoutgenerating surface runoff, and this would have been true also of the formerlycultivated grasslandfor most of the events. However, the hydraulic conductivityvalues at depths below 150 em are low compared with the infiltration rates,indicating that, water must preferentially move laterally to the streams throughthe upper part of the soil profile .. Since the soil conditions are similar under allthe land uses, it appears that at Horton Plains land use has only a marginaleffect on the runoffprocess. compared with soil and climate.