Changing Soil Hydrology Due to Rain Forest Logging: an Example from Sabah Malaysia

Rain forest logging generally involves substantial changes in soil hydrology which lead to accelerated erosion. Many studies consider surface effects, focusing on increased runoff generation resulting from reduced soil permeability at saturation. Changes in soil profile hydrology are wide ranging and have a wider variety of consequences than simply a reduction in soil permeability. Soil moisture retention, saturated soil moisture content and hydraulic conductivity in the unsaturated zone influence moisture movement through the profile, pore water pressure distributions and moisture availability to plants between rain storms. Soil erosion at the surface is affected, but such factors also control the propensity for mass movement and the likely vegetation regeneration following abandonment of logged areas. This paper explores this range of hydrological properties and considers in detail some of the consequences of changing hydrological behaviour for an area of eastern Malaysia which is currently undergoing extensive logging. The results indicate that as well as a decline in saturated permeability, the soils undergo changes in moisture retention which affects unsaturated zone hydrology. The general model of Campbell is fitted to the data, the results of which indicate substantial changes in the relationship between moisture retention, hydraulic conductivity and soil suction following logging. These results form an essential requirement for the application of physically-based hydrology models which may elucidate in detail processes resulting from rain forest disturbance.