HYDROLOGIC AND SALINITY CHANGES ASSOCIATED WITH TREE PLANTATIONS IN A SALINE AGRICULTURAL CATCHMENT IN SOUTHWESTERN AUSTRALIA

Most of a naturally forested catchment (rainfall 760 mm/year) was cleared for agriculture. A saline seep developed, and to reclaim it, 11% of the catchment was replanted to two eucalypt plantations within and above the seep. Water levels and contents, outflow of water and salt, and changes in salinity and plant species in the seep were monitored for several years. Total evaporation by farm vegetation and the plantations, and rainfall interception by plantation and remnant forest trees, were measured. Profiles of root length and of saturated hydraulic conductivity were obtained. Salt-sensitive agricultural species eventually re-established in the saline seep following a small reduction of salt in the top 10 cm of soil. The plantation established upslope did not lower the water table even though evaporation from it exceeded that from pasture-crop by 1600 mm/year. The plantation in and above the seep lowered the water table by 0.5 m. The ratio of chloride in discharge over the weir to chloride in rain increased from unity in 1977 to 17 in 1983, then fell to 1 in 1984, when measurement ceased. Between 1977 and 1983, leaching of salt by lateral (downslope) drainage had occurred under pasture-crop in the 1.5 to 2.4 m depth range. Concurrently, chloride increased under plantations. Soil water profiles showed that the plantation upslope used more water than the one at midslope. Water use by the remnant forest areas assisted in the reclamation of the seep.