Flow systems, tree plantations, and salinisation in a Western Australian catchment

Abstract

A lower hillslope in the Western Australian wheatbelt had become waterlogged and saline by 1981, when close-spaced rows of eucalypts were planted in blocks both in and adjacent to the discharge area and piezometers were established on the site. We analysed the trends in the piezometric 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 vegetation in the upland area would affect the waterlogging and salinity.

Piezometric levels under the trees decreased for the first 5 years after planting and then stabilised until 1991 when they started gradually decreasing again. The non-treed area between the plantation blocks remained unaffected until about 1991, when the levels there also started to decrease quite significantly, probably because of the trees. The trees therefore appear to have been effective and beneficial in the short to medium term.

However, the salinity of the groundwater under the trees 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 health 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 term management strategy, not a solution, and the only way to control salinity in the long term is to plant vegetation species in the recharge areas that use all of the water that falls there. Modelling showed that only a small surplus of water over winter, in the order of 50 mm/year, caused the increased recharge and consequent salinisation. The modelling results also show that the surplus could be managed with an effective vegetation species (e.g. lucerne) with a rooting depth of about 1·5 m that would be able to transpire at least until early to mid summer.