FLOW SYSTEMS, TREE PLANTATIONS, AND SALINIZATION IN A WESTERN-AUSTRALIAN CATCHMENT

A lower hillslope in the Western Australian wheatbelt had become water logged and saline by 1981, when close-spaced rows of eucalypts were pl anted in blocks both in and adjacent to the discharge area and piezome ters were established on the site. We analysed the trends in the piezo metric heads and salinity concentrations over the period of record. We also modelled the hillslope profile using finite element analysis to determine the water flow mechanisms and to see how a change in vegetat ion in the upland area would affect the waterlogging and salinity. Pie zometric levels under the trees decreased for the first 5 years after planting and then stabilised until 1991 when they started gradually de creasing again. The non-treed area between the plantation blocks remai ned unaffected until about 1991, when the levels there also started to decrease quite significantly, probably because of the trees. The tree s therefore appear to have been effective and beneficial in the short to medium term. However, the salinity of the groundwater under the tre es has increased significantly in the last 5 years, particularly where the tree density is highest. The continued flow of saline groundwater to the trees is believed to be increasing the salinity. It could not be expected that plantations of this type will maintain hearth and be able to control the excess water in such an hydrologic setting in the long term. Tree plantations on discharge areas are a short to medium t erm management strategy, not a solution, and the only way to control s alinity in the long term is to plant vegetation species in the recharg e areas that use all of the water that falls there. Modelling showed t hat only a small surplus of water over winter, in the order of 50 mm/y ear, caused the increased recharge and consequent salinisation. The mo delling results also show that the surplus could be managed with an ef fective vegetation species (e.g. lucerne) with a rooting depth of abou t 1.5 m that would be able to transpire at least until early to mid su mmer.