Abstract
A small (344ha) experimental catchment in southwest Western Australia was partially deforested (western 53% of the catchment) in 1976 to study the effects of agricultural development on water quantity and quality. The impact on the groundwater system in the cleared area was dramatic. Initial rates of rise were only 0.11 m year−1 but this increased after 10 years to average 2.3 m year−1. Groundwater rises of 15 m in the valley and 20–25 m on the lower sideslopes were observed over 13 years. A small seep (groundwater discharge area) appeared for the first time in 1988 and by 1989 it covered an area of 1 ha. Streamflow initially increased by 30 mm year−1 (4.0% rainfall) compared with a native forest average streamflow of 8 mm year−1 (1.0% rainfall). However, since the seep area developed, the increase in streamflow has been 50 mm year−1 (6.6% rainfall). Stream salinity was low prior to clearing (30 mgl−1Cl−) and remained low for 9 years after clearing. However, since 1987, stream salinity increased dramatically as the ground water approached the ground surface, and by 1989 reached an annual average of 290 mgl−1Cl−. The daily maximum in 1989 was 2200 mgl−1Cl− compared with 92 mgl−Cl− from 1976 to 1986. The catchment changed from net salt accumulation pre-clearing to net salt export after 1987. Thirteen years after clearing, the groundwater level, stream yield, stream salt load and stream salinity had not reached equilibrium but were all still increasing.
