Hillslope hydrology in tropical rainforest steeplands in Brunei

Many remaining areas of tropical rainforest in south-east Asia are located on landscapes dominated by deep valleys and very steep slopes. Now that log ging activities are extending into these steeplands, it is essential to und erstand how the natural rainforest system behaves if any kind of realistic assessment of the effects of such disturbance is to be made. This paper exa mines the hydrological behaviour of an undisturbed rainforest system on ste ep topography in the Temburong District of Brunei, north-west Borneo. The p hysical and hydrological properties of the regolith material are generally typical of tropical residual soils. The regolith has a day texture and a lo w dry bulk density beneath a superficial litter/organic horizon. The infilt ration capacity of the surface soil was several hundred mm h(-1). That of t he exposed mineral subsoil was an order of magnitude less, similar to the s aturated hydraulic conductivity (K-sat) of around 180 mm h(-1) at a depth o f 150 cm. There was no indication that K-sat reduced with depth except very near the bedrock interface. Soil tensions were measured using a two-dimensional array of tensiometers o n a 30 degrees slope. During dry season conditions, infiltrating rain-water contributes to soil moisture, and drying of the soil is dominated by trans piration losses, During wet season conditions, perched water tables quickly develop during heavy rainfall, giving rise to the rapid production of retu rn flow in ephemeral channels. No infiltration excess or saturation overlan d flow was observed on hillslopes away from channel margins. Subsurface sto rm flow combined with return flow produce stream how hydrographs with high peak discharges and very short lag times. Storm event runoff coefficients a re estimated to be as high as 40%. It is concluded that the most distinctiv e feature of the hydrology of this 'steepland rainforest' is the extremely 'flashy' nature of the catchment runoff regime produced by the combination of thin but very permeable regolith on steep slopes. Copyright (C) 2000 Joh n Wiley & Sons, Ltd.