Effect of Stand Density on Water Use of Mulga (Acacia aneura F.muell.) Woodlands in South-Western Queensland

Abstract

The important Australian fodder tree mulga (Acacia aneura F. Muell.) was thinned to densities ranging between 0 and 4000 trees ha^-1 near Charleville, Queensland. Water use by the ensuing communities was studied from December 1971 to October 1973. Evapotranspiration over this 22 month period increased with tree density from 814 mm for the totally cleared plots to 852 and 891 mm for those plots supporting 4000 and 640 trees ha^-1 respectively. Regression analysis was used to relate soil moisture to time since rain; it was established that the soil had dried to its permanent wilting point within 60 days of rain sufficient to recharge the soil to a depth of 135 cm. The mean apparent maximum evapotranspiration of the mulga communities was 4.7 mm day^-1.

The ratios of evapotranspiration to pan evaporation (E_t/E_o) were influenced by both soil moisture status and tree density. E_t/E_o ratios ranged between 0.55 and 0.07 in the summer of 1972, and between 1.05 and 0.05 the following summer. In the winters of 1972 and 1973 E_t/E_o ratios ranged between 0.92 and 0.04, and 1.33 and 0.09 respectively.

The significance of the results to production and management of mulga lands in south-western Queensland is discussed.