There are many popular misconceptions about the hydrological behaviour of rainforests and the impact of forestry activities on these aquatic processes. In part, this is because large natural variability (in space and time) makes it difficult to either extrapolate local observations to the landscape-scale, or identify purely anthropogenic change. To overcome these difficulties, work within tropical rainforests requires application of the best field and modelling technologies, not those that are easiest to resource.

One site where these fundamental issues are being addressed is the Danum Valley region of the Malaysian State of Sabah. Here, on Borneo Island, a group of Malaysian and UK scientists (notably Waidi Sinun, Ian Douglas and Tony Greer) established hydrological monitoring in 1986, as part of a joint enterprise between the Sabah Foundation and the Royal Society of London. The first projects were focused on the potential impacts of the first phase of selective, commercial forestry on the hydrology of lowland rainforest (Douglas et al., 1992 Phil. Trans. Roy. Soc. Lond. B.). This study and subsequent ecohydrological projects established a series of experimental catchments, ranging in size from the 44 ha Baru catchment to the 721 km sq. Segama catchment.

Analysis of the most recent Danum Valley data has shown that the local rainfall regime conforms to that expected of Equatorial region with relatively little annual seasonality and a dominance of short-duration, convective storms that generate flashy river behaviour. Over distances of only a few kilometers, the rainfall exhibits a very high degree of spatial variability, being strongly moderated by the combined effects of monsoonal wind direction and topography (Bidin, 2001 PhD thesis). Additionally, El Niño Southern Oscillation (ENSO) phenomena give rise to cycles in the rainfall (Chappell et al., 2001 Plant Ecology) that become magnified in the river sediment records. Magnified partly by the changing incidence of extreme events (Douglas et al., 1999 Phil. Trans. Roy. Soc. Lond. B.; Chappell et al., 2002 CUP-UNESCO).

Less than 5% of the incoming rainfall generates surface-flow away from stream channels, with almost all riverflow being generated only after rainfall has entered the ground. This flow is strongly related to the preferential pathways of natural soil pipes, percoline zones and rock fractures. Such routes are poorly characterised by traditional methods, and their presence in the Danum catchments has lead to the development of new whole-hillslope, hydraulic tests (Chappell et al., 1998 Hydrological Processes). The role of such preferential or localised phenomena is also seen within the erosion / sediment system. The nested catchment structure utlised within the Baru, clearly demonstrated that over the long-term, a few landslides and smaller collapses were the key source of soil particles flowing down river channels (Chappell et al., 1999 Phil. Trans. Roy. Soc. Lond. B.). While soil-slope instabilities were observable within the undisturbed forest of the Danum Valley Conservation Area (Balamurgan, 1997 PhD thesis; Chappell et al., 1999b Soil Till. Res.; Tangki, 2001 MPhil) the incidence of collapses along timber-lorry, haulage roads seemed particularly high. The greater role of landslides (triggered by changing subsurface-water conditions) relative to canopy disturbance and surficial erosion along skidder-vehicle trails may have important implications for the revision of current, sustainable forestry guidelines. Canopy disturbance did, however, impact on the evaporative transfers to the atmosphere. Surprisingly, greater rainfall reached the ground below undisturbed remnants of rainforest, relative to areas with a high degree of canopy change (Bidin 2001 PhD thesis). Such changes may be offset by reduced transpiration losses following disturbance and this is the focus of a series of new projects.

The role of extreme events, and the changing patterns of evaporation and erosion with forest recovery, underline the importance of continued monitoring at one of the few reference sites for lowland tropical rainforest.