Hydrology
The hydrology of the area has been extensively researched since 1985, with long-term catchment monitoring stations on two small catchments (one in the primary forest of the conservation area, the other in forest selectively logged in 1988/1989) and on the Segama River. Investigations have also been made of interception, runoff processes and slope erosion. River catchment water balance calculations indicate that annual evapotranspiration is very high (e.g. 1089 mm for the logged forest catchment in 1995-96, and a little higher in primary forest). This is due mainly to high transpiration as annual interception losses are comparatively low, reflecting the low storage capacity of the predominantly leathery and smooth forest leaves, high rainfall intensities and the fact that a significant proportion of rainfall is accounted for by large rainstorms (in which only a small percentage of rainfall is intercepted). Interception in logged forest is somewhat higher than in primary forest, but because of very low interception in forest gaps (log-landing areas, logging roads, skid trails), interception is reduced overall.
In the primary forest, soil infiltration capacities and subsoil permeabilities are high and widespread saturation of soils on slopes does not occur. The rapid response of streams in storm events is a result of a combination of pipeflow, localised saturation, overland flow around streamhead hollows and some Hortonian (infiltration-excess) overland flow. Some Hortonian overland flow (approx. 4-6 % of rainfall) is generated as a result of a combination of a fine surface root mat and matted leaf litter, but most water infiltrates even in intense rainstorms. Of more significance is pipeflow (flow through soil tunnels about at about 80 cm depth), which is generated quickly in storms because of rapid vertical transmission of some of the infiltrating water down soil cracks, fissures left by decayed roots, animal burrows and other macropores. As a result streams respond very quickly to rainfall.
Erosion rates are high by rainforest standards, reflecting the relatively erodible soils and sedimentary rocks of the region. Drainage density of the area is high (20-22 km/km2) for the same reasons. Suspended sediment yields in primary forest are 300-400 t km-2 a-1. Long-term measurements of ground-level change over the period 1990-2002 in the primary forest indicated a mean lowering rate due to slopewash (erosion by overland flow and rainsplash) of 0.36 mm a-1 (or 377 t km-2 a-1). Pipe erosion has also been found to provide a significant proportion of stream sediment loads.
Selective logging tends to lead to a sharp increase in suspended sediment loads in the first two years after logging (e.g. from 342 t km-2 a-1 before logging to a peak of 1633 t km-2 a-1 as recorded in the 1988/89 logged forest catchment close to Danum), an initial marked decline in sediment loads as revegetation occurs; then a secondary peak (592 t km-2 a-1) due to landsliding associated with the decay of bridges and culverts in logging roads and logs in debris dams 5-10 years post-logging, before a renewed decline towards pre-logging rates. Erosion rates can be reduced greatly with careful alignment of roads and through the adoption of Reduced Impact Logging techniques.
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