The role of extreme events in the impacts of selective tropical forestry on erosion during harvesting and recovery phases at Danum Valley, Sabah

Abstract

Ten years' hydrological investigations at Danum have provided strong evidence of the effects of extremes of drought, as in the April 1992 El Niño southern oscillation event, and flood, as in January 1996. The 1.5 km2 undisturbed forest control catchment experienced a complete drying out of the stream for the whole 1.5 km of defined channel above the gauging station in 1992, but concentrated surface flow along every declivity from within a few metres of the catchment divide after the exceptional rains of 19 January 1996. Under these natural conditions, erosion is episodic. Sediment is discharged in pulses caused by storm events, collapse of debris dams and occasional landslips. Disturbance by logging accentuates this irregular regime. In the first few months following disturbance, a wave of sediment is moved by each storm, but over subsequent years, rare events scour sediment from bare areas, gullies and channel deposits. The spatial distribution of sediment sources changes with time after logging, as bare areas on slopes are revegetated and small gullies are filled with debris. Extreme storm events, as in January 1996, cause logging roads to collapse, with landslides leading to surges of sediment into channels, reactivating the pulsed sediment delivery by every storm that happened immediately after logging. These effects are not dampened out with increasing catchment scale. Even the 721 km2 Sungai Segama has a sediment yield regime dominated by extreme events, the sediment yield in that single day on 19 January 1996 exceeding the annual sediment load in several previous years. In a large disturbed catchment, such road failures and logging-activity-induced mass movements increase the mud and silt in floodwaters affecting settlements downstream. Management systems require long-term sediment reduction strategies. This implies careful road design and good water movement regulation and erosion control throughout the logging process.

Influence of watershed‐climate interactions on stream temperature, sediment yield, and metabolism along a land use intensity gradient in Indonesian Borneo

“…Previous work in tropical regions describes elevated sediment export from recently cleared agricultural catchments [Bramley and Roth, 2002;Douglas et al, 1999;Dunne, 1979;Hunter and Walton, 2008]. Similarly, we found that mean sediment yield was greater in the young oil palm stream than in mixed agroforest, logged forest, and intact forest sites (Figure 3).…”

“…Agricultural conversion also affects stream physical and biochemical properties. For example, stream temperatures tend to be higher in agricultural than forested catchments [Macedo et al, 2013;Neill et al, 2013], while sediment concentration and load are typically elevated after land clearing [Bruijnzeel, 2004;Douglas et al, 1999]. Enhanced sediment production is often sustained through activities into recently cleared and planted oil palm (young oil palm, OPY, <3 years post clearing in 2008) and mature oil palm (OPM, >10 years).…”

Quantifying The Rainfall-Water Level Fluctuation Process in a Geologically Complex Lake Catchment

“…Many black box hydrologic models have been successfully applied in the past to simulate processes such as the groundwater discharge in karstic springs and rainfall-runoff relationships in areas of high structural heterogeneity where distributed models is difficult to operate efficiently (Jukic et al, 2003;Labat et al, 2000;Young and Beven, 1994;Chappell et al, 1999;Douglas et al, 1999). A significant advantage of the Transfer Function (TF) black box models is that they are parsimonious, demanding limited input data and comprehension of the system while they facilitate time series analysis of parameters that present high degree of autocorrelation (Niu et al, 1998).…”

Quantifying Fine-Sediment Sources in Primary and Selectively Logged Rainforest Catchments Using Geochemical Tracers

“…Rotational selective logging (where trees above a threshold girth are extracted in annual coupes within a larger forest area, with each coupe left to recover before subsequent selective logging) is a relatively sustainable form of tropical forest exploitation (Marsh & Greer, 1992), but still has major erosional and hydrological impacts from elements of a post-logging disturbance mosaic such as skid trails and primary and feeder logging roads (Douglas et al, 1999;Sidle, Sasaki, Otsuki, Noguchi, & Nik, 2004). Work to date has focussed on direct monitoring of processes on slopes and in small catchments, with little attention given to the downstream transfer and storage of sediment, despite their importance to such issues as siltation, flooding, river pollution, water supply, fish stocks and offshore coral.…”