Catchment water balance is strongly affected by land-use and vegetation characteristics; generally, trees use more water than pasture and crops. As a result, forested catchments yield less stream flow (water yield). In the Murray–Darling Basin (MDB) there are plans to convert large areas of pasture to forestry plantations in the coming decades; a range of commercial and environmental considerations motivates these plans. This chapter describes how a simple water balance model in a geographic information systems (GIS) framework can be used to assess average annual stream flow under different land-use scenarios. The model only requires percentage forest cover of a catchment and mean annual rainfall. The method is well suited for regional-scale assessment of the impacts of change in land use on water yield.

The case study described here is based on average rainfall data for 1980–95, and vegetation cover data under different land-use conditions obtained from the MDB Commission, the Australian Land Information Group (AUSLIG), and CSIRO Forestry and Forest Products. Estimated mean annual catchment water yields agreed well with measured stream flow data for catchments with medium to high rainfall in the MDB but tended to overestimate water yield for low-rainfall catchments. The model showed that clearing native vegetation in the MDB is likely to significantly increase water yield from most of the catchments within the basin. It also predicted that afforestation in the basin may reduce mean annual water yield by up to 40 mm/year.