Interactions between trees and groundwaters in recharge and discharge areas : A survey of Western Australian sites

In Australia revegetation of both groundwater recharge and discharge areas is seen as a fundamental element of dryland salinity management. Millions of trees are planted each year with the intention of controlling rising water tables induced by the clearing and subsequent replacement of native, deep-rooted vegetation with lower water use agriculture systems. These trees are planted in areas with either considerable depths to the water table (recharge areas) or in areas with shallow water tables and high soil salinity levels (discharge areas). To assess how trees established in these areas have altered groundwater conditions, observations from almost 80 sites in south-western Australia have been analysed. Statistical analyses suggest that planting trees in groundwater recharge areas will only lead to significant reductions in water levels if considerable areas of the catchment are planted. In discharge areas, the probability of achieving reduced water levels was lower and the magnitude of the response smaller. Additionally, in discharge areas, the maximum reduction of water tables from revegetation was about 2.5 m and the effect greatest where groundwater salinity was relatively low (<5000 mg 1-1). Best results from revegetation occurred in local-scaled aquifers. Evidence presented in five case studies suggests that if the hydrogeologic conditions are well defined, tree plantations maybe a useful tool for localised salinity management. However, at all but three of the 80 sites reviewed, trees had little or no effect on the water tables more than 10-30 m from the planted area. The exceptions were the Merredin site, where preliminary data suggest that water tables appear to have been lowered by almost I m, 3 km from a large plantation and at the Narrikup and East Belka sites, where the water table was influenced 75 m upslope (Narrikup) and downslope (East Belka) from the planted area. If the objective of the land managers is to prevent the spread of salinity and reclaim existing areas, then only the adoption of broadacre recharge management systems can be effective in the longer term.