Many remaining areas of tropical rainforest in south-east Asia are located on landscapes dominated by deep valleys and very steep slopes. Now that logging activities are extending into these steeplands, it is essential to understand how the natural rainforest system behaves if any kind of realistic assessment of the eects of such disturbance is to be made. This paper examines the hydrological behaviour of an undisturbed rainforest system on steep topography in the Temburong District of Brunei, north-west Borneo. The physical and hydrological properties of the regolith material are generally typical of tropical residual soils. The regolith has a clay texture and a low dry bulk density beneath a super®cial litter/organic horizon. The in®ltration capacity of the surface soil was several hundred mm h À1 . That of the exposed mineral subsoil was an order of magnitude less, similar to the saturated hydraulic conductivity (K sat ) of around 180 mm h À1 at a depth of 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 on a 308 slope. During dry season conditions, in®ltrating rain-water contributes to soil moisture, and drying of the soil is dominated by transpiration losses. During wet season conditions, perched water tables quickly develop during heavy rainfall, giving rise to the rapid production of return ¯ow in ephemeral channels. No in®ltration excess or saturation overland ¯ow was observed on hillslopes away from channel margins. Subsurface storm ¯ow combined with return ¯ow produce stream ¯ow hydrographs with high peak discharges and very short lag times. Storm event runo coecients are estimated to be as high as 40%. It is concluded that the most distinctive feature of the hydrology of this steepland rainforest' is the extremely ¯ashy' nature of the catchment runo regime produced by the combination of thin but very permeable regolith on steep slopes.