The effect of logging and regeneration on groundwater, streamflow and stream salinity in the southern forest of Western Australia

Abstract

In the late 1960s the Forests Department of Western Australia decided to change its logging strategy in the southern forest from light selection cutting to heavy selection cutting and clear-felling. This raised some concern about the effect on the water resources in the region. Four small catchments (Crowea, Poole, Iffley and Mooralup) were therefore selected in 1975 to study the effect of heavy selection cutting and clear-felling followed by regeneration on streamflow, stream salinity and groundwater levels. The catchments were logged between November 1976 and March 1978. Regeneration began within eighteen months after the completion of logging. During the study (1976–1985) the annual rainfall in the region was generally below the long-term mean. This probably influenced the magnitude and duration of the hydrologic response to logging and regeneration, but not the general trends. Groundwater levels rose for two–four years after logging and then started to fall again. They can be expected to reach the values they would have been at without logging within fifteen years after the beginning of regeneration. In the relatively dry Mooralup catchment, logging had little effect on the groundwater level. As a result of logging streamflow increased for two years (1977 and 1978) and then gradually declined again as the vegetation regenerated. Streamflow is also likely to be back to pre-logging values within fifteen years after the beginning of regeneration. In the Mooralup catchment, where streamflow volumes are naturally small, this may have happened after six years of regeneration. Flow-weighted mean annual stream salinities rose for one–three years after logging but have declined since. Even at their highest level they remained below the upper limit for high-quality drinking water. Stream salinites are likely to return to the level they would be at without logging at the same time as streamflow and groundwater levels.