John Ruprecht

Bauxite mining occurs over an extensive area of the northern jarrah forest within water supply catchments for Perth and the south-west region. The hydrologic impact of mining and subsequent rehabilitation have been monitored at two experimental catchments, More Seldom Seen and Del Park. Based on comparison to native forest control catchments the groundwater, streamflow and stream salinity response to mining were observed. The major response to mining was a significant increase in streamflow. The maximum streamflow increases were 8-20% of annual rainfall (160-240 mm), and were related to the area of catchment mined. Groundwater levels rose by a maximum of 2-4 m. Both groundwater levels and streamflow increases declined as the mined areas were rehabilitated. There was a slight increase in stream salinity. However it was not significant. The hydrologic impact of mining and rehabilitation was of similar magnitude as that of temporary land use changes such forest logging or thinning in south-west Australia.

Soil water dynamics were measured on a hillslope transect in the jarrah forest of south-west Western Australia over the period 1984-86. The annual maximum to minimum unsaturated soil water storage was approximately 480 mm. This did not change significantly in years of substantially different winter rainfall. The depth of infiltration was dependent on annual rainfall, with groundwater recharge occurring over the whole hillslope in wet years but confined to the lower slopes in dry years. The average summer drying rate of the soil profile to 6 m depth was 3.5 mn day-1. In late summer the drying rate decreased downslope but increased midslope. Two relationships between streamflow to rainfall ratio and soil water storage were identified, the first from May to September and the second from October to December.