Abstract

There are very few experimental studies identifying hydrological pathways within rain forest slopes. Such knowledge is however, necessary to understand why forest disturbance affects rainfall-riverflow response and nutrient migration. This study examines flow pathways within lowland rain forest slopes comprising of Udults of the Ultisol soil order. Experimentation was conducted on four SE Asian hillslope units (each 5 x 5 m in plan) in the Bukit Timah catchment (Singapore Island), and in the W8S5 catchment (Sabah, Borneo Island). The flow pathways were identified by artificial tracer experiments. How well hydrometric calculations based on tensiometry and permeametry measurements predicted the tracer patterns was evaluated.

The tracer work indicated much faster subsurface flows at Bukit Timah than W8S5 for the storms studied. Some explanation of the greater subsurface waterflows at Bukit Timah in comparison to W8S5 is afforded by the less steep moisture release curves which maintain hydraulic conductivity as the soil dries. Vertical flow of the tracer through the upper one metre of soil predominated (>90 percent of percolation) in the Bukit Timah slopes. In some contrast, a major component (approximately 60 percent) of the tracer percolation was directed laterally within the W8S5 slopes. The flow vectors calculated using the hydrometric methods did however, grossly under-estimate the degree of lateral deflection of waterflow generated at W8S5 and to a lesser extent over-estimated it at Bukit Timah. In part, these errors may relate to the inability if traditional hydrometric techniques to fully characterise the effect of the large and small ‘natural soil pipes’ present within both catchments. In conclusion, the study indicates that marked variations in flow vectors exist within the Udult great group of SE Asian soils and hydrometric calculations may be poor predictors of these dominant pathways.