Streamflow and stream salinity in a small water supply catchment in southwest Western Australia after reforestation

Abstract

In the mid-1970s high salinities developed in the Padbury reservoir in southwest Western Australia due to the conversion of 80% of its catchment from native forest to farmland in previous decades. Between 1977 and 1983 some 70% of this farmland was planted with pines and eucalypts. Most of the slopes were reforested, but only some of the valleys. In 1978 a gauging station was established in the catchment to monitor the effect of reforestation on streamflow and stream salinity. Results obtained by the end of 1986 are presented here. Reforestation led to a substantial and continuous reduction in streamflow. There was also a steady reduction in salt discharge from the catchment. In most years the reduction in streamflow outweighed the reduction in salt discharge. Stream salinity was therefore generally higher than it would have been without reforestation. Since 1978, when hydrologic monitoring began, the annual rainfall was generally below the long-term mean of 880 mm, and often below 800 mm. While these dry conditions were at least partly responsible for the increases in stream salinity, it is not known whether salinity would have decreased in all years if the rainfall had been normal. It is argued that completely replanting the valleys would have resulted in lower stream salinities. If rainfall had been normal, or if the valleys had been planted, and particularly with a combination of the two, reforestation may have resulted in a general decrease in stream salinity. Yet, any salinity benefit due to reforestation must be balanced against the concurrent reduction in streamflow. For a small water supply catchment like the one described here, reforestation may therefore not be an appropriate strategy to alleviate salinity problems. However, it is shown that reforestation is useful in large salt-affected catchments in the region where the areas which contribute most of the salt generate only little of the streamflow.