Abstract

The links between catchment geomorphology and hydrology in tropical catchments is poorly understood. Available literature for the forested humid tropic catchments suggests that hillslope flow pathways vary considerably, attributable to such factors as soil type, catchment geomorphology, lithology and rainfall characteristics. We report on a study from two tropical rain forest catchments in Southeast Asia that seek to characterize contrasting water flow pathways through different soil types: the Ferric Acrisol of the Jungle Falls catchment (Singapore), and the Haplic Alisol of the W8S5 catchment (Malaysia). Potential gradient data over the upper 90 cm of the soil profile were derived from 25 multi-depth tensiometers installed across a distribution of 5x5 m plots. These were combined with hydraulic conductivity data, to determine the magnitude and direction of the Darcian flux. Tracer experiments were also conducted within the plots, whereby 5.8 kg sodium chloride was applied as a line source directly across the upper portion of each plot. Our results indicate that flows within the Ferric Acrisol were predominantly vertical to depth, despite evidence of near-surface saturation during intense rainfall. Flux was predominantly lateral through the near-surface A and B1 horizons of the Haplic Alisol despite the fact that there was no significant difference between the measured saturated hydraulic conductivity of the two horizons (this may be explained by ring permeametry error, which was highly sensitive to the artificial boundary conditions imposed on the excavated soil core). Notwithstanding, we show that the contrasts in flow behavior between the two catchments is related to the very different hydraulic conductivity distributions associated with the physical and chemical composition of the soils.