Early hydrological response to intense forest thinning in southwestern Australia

A small forested catchment in southwest Western Australia was thinned to study the effect on hydrology, wood production and disease escalation. This paper deals primarily with the hydrological aspects. The uniform, intensive thinning treatment reduced crown cover from 60 to 14%, which resulted in an increase in streamflow of approximately 20% of annual rainfall (260 mm for an average year) after 3 years, compared with a streamflow yield of 6% of annual rainfall before thinning. The deep groundwater attained a new equilibrium after 2 years, rising by approximately 2 m in the area adjacent to the swamp, and by 5 m upslope. The ephemeral shallow groundwater system expanded in duration from 2 months per year pre-treatment to approximately 6 months per year after thinning. The major components of streamflow generation present before thinning remained as the major components after thinning. The expansion of the saturated source area and the presence of a shallow groundwater system for extended periods, as a result of an increase in available water from the reduction in interception and evaporation from the overstorey, were considered to be the major causes of increased streamflow.