Streamflow and Salinity Response to Agricultural Clearing at the Warren River Basin, Western Australia

Abstract

The Warren River Basin is one of the largest water resources systems in the south-west of Western Australia. It contains 16% of total potable resources in the south-west Drainage Division. Stream salinity has increased due to clearing of native forest for pasture development. In 1978, the State Government enacted Legislation to control large scale agricultural clearing within the catchment as an attempt to control and improve stream salinity.

The main gauging station, located at Barker Road Crossing has a catchment area of 4022 km². The climate of Warren River basin is Mediterranean with warm, dry summers and cool, wet winters. Average annual rainfall is 865 mm which ranges from 1400 mm near the coast to 500 mm at the inland divide. Annual pan evaporation averages 1275 mm About half of the river basin is in State Forest. Most of the other half had been cleared through forestry operations or agricultural development. After Clearing Control Legislation was introduced in 1978, the demand for agricultural clearing had diminished by the end of 1994.

Annual streamflow ranged from 21 mm to 344 mm, depending upon annual rainfall and vegetation cover of the catchments. However, the main gauging station of Warren River, located at Barker Road Crossing, had an annual average streamflow of 72 mm. The trends in annual streamflow were different for the various rainfall regions of the catchments. In the Low Rainfall Zone, where most of the sub-catchments were extensively cleared, there was a continuous and significant increase in streamflow. The highest rate of annual streamflow increase of 0.75 mm was observed at Tone River. In the Intermediate Rainfall Zone, streamflow increase was not significant. However, there was a significant rate of decrease in streamflow in the High Rainfall Zone.

Stream salinity ranged from 160 mgL^-1’ TSS in the High Rainfall Zone to 4900mgL^-’ TSS in the Low Rainfall Zone. The average stream salinity of Warren River at Barker Road Crossing was 785mgL^-’ TSS, with a statistically significant increasing rate of 10 mgL^-’yr^-1 TSS. The stream salt load of Warren River at Barker Road Crossing was 525 kgha^-lyr^-1. Annual increase in salt load was significant at 7.6 kgha^-’yr¹. The stream salinity and salt load trends of the some of the sub-catchments were not statistically significant. The highest salt discharge of 1161 kgha^-’yf¹ was from Tone River sub-catchment. The Tone River contributes 11% of flow and 54% of salt load to the total for Warren River at Barker Road Crossing.

From a regional water resources perspective, stream salinity of the Warren River Basin is increasing. To combat stream salinity increase, a catchment management strategy should be developed and promoted. This should involve a more systematic management of land use to meet specific goals of water quality, land productivity, conservation and recreational values.