HYDROLOGICAL RESPONSE TO THINNING A SMALL JARRAH (EUCALYPTUS-MARGINATA) FOREST CATCHMENT

The jarrah (Eucalyptus marginata) forest on a small catchment was thin ned early in 1983 to study the effect on catchment hydrology. The thin ning reduced canopy cover, basal area and stocking about two-thirds. R ainfall during the pre and post-treatment periods was 21% and 10%, res pectively, below the long-term average. Streamflow increased from 0.5% of rainfall (4.3 mm) before thinning to 7.6% of rainfall (90 mm) 9 ye ars after thinning. Streamflow duration increased, with the largest in creases in streamflow in the wet winter months of June-October. The de ep groundwater level at a midslope location increased by 8 m and at a valley location by 4 m in the 8 years after thinning. There was no ind ication of a new equilibrium being reached for either streamflow or gr oundwater, thus further increases in streamflow and groundwater level are likely.