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Increases in the frequency of large to extreme rainfall events are widely predicted with global warming, but evidence from the humid tropics is hampered by a paucity of long-term data. This paper a...

Long-term (21–30 years) erosional responses of rainforest terrain in the Upper Segama catchment, Sabah, to selective logging are assessed at slope, small and large catchment scales. In the 0.44 km 2 Baru catchment, slope erosion measurements over 1990–2010 and sediment fingerprinting indicate that sediment sources 21 years after logging in 1989 are mainly road-linked, including fresh landslips and gullying of scars and toe deposits of 1994–1996 landslides. Analysis and modelling of 5–15 min stream-suspended sediment and discharge data demonstrate a reduction in storm-sediment response between 1996 and 2009, but not yet to pre-logging levels. An unmixing model using bed-sediment geochemical data indicates that 49 per cent of the 216 t km −2 a −1 2009 sediment yield comes from 10 per cent of its area affected by road-linked landslides. Fallout 210 Pb and 137 Cs values from a lateral bench core indicate that sedimentation rates in the 721 km 2 Upper Segama catchment less than doubled with initially highly selective, low-slope logging in the 1980s, but rose 7–13 times when steep terrain was logged in 1992–1993 and 1999–2000. The need to keep steeplands under forest is emphasized if landsliding associated with current and predicted rises in extreme rainstorm magnitude-frequency is to be reduced in scale.

Detailed information on post-logging sediment dynamics in tropical catchments is required for modelling downstream impacts on communities and ecosystems. Sediment tracing methods, which are potentially useful in extending to the large catchment scale and longer time scales, are tested in primary and selectively logged rainforest catchments of Sabah, Borneo. Selected nutrient (P and N) and trace metal (Ni and Zn) concentrations are shown to discriminate surface, shallow subsurface and deep subsurface sediment sources. Analysis of channel-stored fine-sediment samples and use of an unmixing model allow the relative importance of these vertical sediment sources to be estimated and erosion processes to be inferred for catchments of contrasting size.

R.P.D. Walsh1, M.A. Clarke2, K. Bidin3, W.H. Blake4, N.A. Chappell5, I. Douglas6, N. Ramli3, A.M. Sayer1, W. Sinun7, J. Larenus8 and J. Hanapi8 1University of Wales Swansea, UK; 2National Soils Research Institute, UK; 3Universiti Malaysia Sabah, Malaysia; 4University of Plymouth, UK; 5Lancaster University, UK; 6Manchester University, UK; 7Innoprise Corporation Sdn. Bhd., Malaysia; 8Danum Valley Field Centre, Malaysia

Despite soil piping being increasingly reported from the humid tropics, quantitative assessments of the role of pipeflow in runoff generation and sediment supply in humid tropical catchments remain lacking. This paper assesses pipeflow and streamflow suspended sediment dynamics in small humid tropical rainforest catchments in Danum Valley, Sabah, Malaysian Borneo. Pipeflow and streamflow in two small catchments were continuously monitored between December 2002 and June 2004 using pressure-transducer probes behind V-notch weirs; suspended sediment was continuously monitored with turbidity probes. Monthly flow and sediment load data show that a single monitored pipe in one catchment accounted for 47.2% of a stream's discharge and 21.6% of the suspended sediment load over the monitoring period. Anecdotal evidence from unmonitored pipes suggests the total contribution from pipes to streamflow discharge and sediment loads may be much higher. In general, absolute monthly pipe and stream sediment yields increase with monthly rainfall and pipe or stream discharge. Fourteen storm event responses are considered in detail. Pipe and stream discharge-suspended sediment responses to storm events are rapid and are characterised by clockwise hysteresis. An inverse relationship between the percent contribution of sediment to streamflow from a monitored pipe and rainfall intensity was identified, indicating that in large, intense rainstorms other sediment sources (including slopewash and unmonitored pipes) become more important. Mechanisms of pipe discharge and suspended sediment generation are discussed and implications for future runoff, sediment and channel development models are briefly outlined.