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Impacts of native forest harvesting on flows into the Murray–Darling Basin system

Recent drought in Australia has shown the importance of inflow into the Murray–Darling Basin from forested catchments in high-rainfall areas. Concern has been expressed that native forest logging could, by leading to regrowth forests, diminish this inflow. This reflects that at least one eucalypt community has a water use in which regrowth water yield is, for a given annual rainfall, less than that from ‘old-growth’ forest. Available data sets showing the relation between forest age, annual rainfall and water yield from forested catchments were used to select the best of four models of age-related water yield from forests. Commercial forests across the entire Murray Darling Basin were categorised by productivity and by location in rainfall bands. Using assumptions as to the distribution of age classes, the impact of native forest harvesting on water yields was compared with having all ‘old-growth’ in these forests. The results showed that for most of the Murray–Darling Basin, native forest logging would increase water yields by a small amount. Areas with potential for decreasing yield were the high-rainfall catchments of the Goulburn–Broken River, the Ovens–Kiewa River and the Upper Murray River. The results show that a cessation of harvesting would lead to a small decrease in flow before flows increased after about twenty years. Potential gains in water yield could be substantial but would take long periods to achieve and would be dependent on a lack of natural disturbance that could result in forest regeneration. The analysis showed that there is probably an optimal level of wood and water production, but the data are inadequate to determine that. There is a need for considerable work in forest hydrology if such questions are to be answered.