Impacts of tree plantations on groundwater in south-eastern Australia

In some regions dependent on groundwater, such as the lower south-east of South Australia in the Green Triangle, deep-rooted, woody vegetation might have undesirable hydrological impacts by competing for finite, good-quality groundwater resources. In other regions, such as the Riverina in south-central New South Wales, where rising watertables and associated salinisation is threatening the viability of agriculture, woody vegetation might have beneficial hydrological impacts. In response to a growing need to better understand the impacts of tree plantations on groundwater, annual evapotranspiration and transpiration were measured at 21 plantation sites in the Green Triangle and the Riverina. Sources of tree water uptake from rainfall and groundwater were determined by measurements of evapotranspiration and soil water over periods of 2 - 5 years. In the Green Triangle, under a combination of permeable soil over groundwater of low salinity (< 2000 mg L-1) at 6-m depth or less, in a highly transmissive aquifer, annual evapotranspiration at eight research sites in Pinus radiata D. Don and Eucalyptus globulus Labill. plantations averaged 1090 mm year(-1) ( range 847 - 1343 mm year(-1)), compared with mean annual precipitation of 630 mm year(-1). These plantation sites used groundwater at a mean annual rate of 435 mm year(-1) ( range 108 - 670 mm year(-1)). At eight other plantation sites that had greater depth to the watertable or a root-impeding layer, annual evapotranspiration was equal to, or slightly less than, annual rainfall ( mean 623 mm year(-1), range 540 - 795 mm year(-1)). In the Riverina, where groundwater was always present within 3 m of the surface, Eucalyptus grandis Hill ex Maiden trees at three sites with medium or heavy clay, alkaline, sodic, saline subsoils used little or no groundwater, whereas E. grandis and Corymbia maculata ( Hook.) K. D. Hill and L. A. S. Johnson trees at a site with a neutral sandy soil and groundwater of low salinity used 380 and 730 mm year(-1) of groundwater ( respectively 41 and 53% of total annual evapotranspiration). We conclude that commonly grown Eucalyptus species and P. radiata are able to use groundwater under a combination of light- or medium-textured soil and shallow depth to a low-salinity watertable.