Runoff generation and re-distribution in logged eucalyptus forests, south-eastern Australia

Abstract

While pristine forests are traditionally regarded as environments with low runoff and low erosion potential, timber harvesting can dramatically affect surface runoff production, with some important consequences for in-stream water quality. A series of large-scale (300m2) rainfall simulator experiments on disturbed forest hillslopes, incorporating both snig track (skid trail) and general harvesting areas (GHA), examined runoff generation and redistribution during three sequential simulated storms on 13 sites. The simulated storms represented 30-min rainfall events with recurrence intervals of 2, 10, and 100y respectively. The 13 sites were selected to represent dominant soil types and ages since disturbance. The snig tracks and GHA are characterised by significantly different soil hydraulic and vegetation properties as reflected in the nature and magnitude of runoff production. Infiltration-excess runoff dominated the snig track areas, while on recently disturbed GHA, runoff was distinctly patchy because of the high degree of spatial variability in saturated hydraulic conductivity (Ks) and degrees of disturbance. Surface runoff from recently constructed snig tracks was an order of magnitude higher than the neighbouring GHA. Relative differences in runoff production between the two areas declined during extreme rainfall events. Infiltration tests also indicated that a change in the processes and rates of runoff persists for at least 5y after disturbance. The practice of redistributing concentrated snig track runoff at cross banks was most effective for small storms. However, during more extreme events, the volume of snig track runoff increased and flow velocities and transport distances increased as the hillslope became increasingly saturated. The varying ability of the GHA to generate runoff and absorb concentrated flow from the snig track is likely to be critical in predicting both the initial hydrologic response and the recovery of a small catchment from forest disturbance.