Abstract
Five small forested catchments in the Collie River Basin in southwest Western Australia were instrumented to quantify the changes in the input and output of salt and water when land use changed from forest to agriculture. The paired catchment approach was used with two adjacent catchments located in high rainfall (1200 mm yr−1) and a group of three located in a lower (800 mm yr−1) rainfall area. The treatment of complete clearing was applied to one catchment in the high rainfall zone, and to about half of one catchment in the lower rainfall zone after three years of calibration with the uncleared control catchments. Strategies of partial clearing were applied to the third catchment in the lower rainfall zone. The catchment pairs were found to be well matched.
The yield of water and salt increased immediately after clearing. In the higher rainfall catchment streamflow increased about four times to 48% of rainfall, salt load increased to fifteen times the mean saltfall (measured as chloride ion) of 7 g m−2 yr−1. Direct runoff increased fourfold to 16% of yearly streamflow, due partly to the progressive increase in the area of soil saturated to the surface. The perched groundwater system was the major source (77% in 1983) of streamflow though it was the route rather than the source of increased salt to the stream. The mottled and pallid clay profile was the dominant source of salt transported by the deep groundwater system into the perched aquifer. Streamflow became perennial in the first postclearing year.
Absolute increases in streamwater and salt output from partially cleared catchments in the lower rainfall location were about an order of magnitude less than for the higher rainfall catchment though the relative increases were similar. The three catchments were found to be accumulating salt. In the cleared catchments the rising potentiometric surface would establish a saline groundwater discharge zone in the valley and a net export of salt.
In all treated catchments reduced interception results in increased water availability at the soil surface for stormflow and infiltration. The reduction in transpiration had an added effect in increasing recharge to groundwater. A new equlibrium for water had not been achieved eight years after clearing, but was estimated to precede the new salt equilibrium.
