Résumé
Water logging and salinity are major forms of land degradation in southwestern Australia and are predicted to become even more extensive. There are basic requirements for the development of salinisation and water logging within the specific geology and geomorphology of the region. The nature, extent and consequences of water logging and secondary salinity in the agricultural areas of southern Australia is presented in overview with a particular emphasis on the type of landscape represented by the ‘Ucarro’ experimental site, near Katanning in southwestern Australia.
Three types of water logging are identified. These are associated with perched aquifers in texture-contrast (duplex) soils, inundation of terraces and valleys, and saturation in surface soil due to the hydraulic pressure being above ground level in aquifers at the base of the regolith of highly weathered granites and gneisses or within channels in broad valley sediments. A dominance of the vertical component of flow in perched aquifers is identified which contributes significantly to recharge of deep aquifers and subsequent land salinisation. Water logging has reduced the potential yield by 30–80% for many crops and pastures in the >400
mm rainfall zone. Increasing land salinisation has been accompanied by increasing trends in salinity of water resources.
There is an interaction between water logging and salinity where groundwater discharge occurs at the soil-surface. Water logging may be the more serious inhibitor for plant growth than salt even at low salt levels. The interaction of water and salt is synergetic for plants. Remotely-sensed techniques using surrogates such as vegetation type and condition have been developed to provide broad spatial assessments of saline and water-logged areas since 1987. In 1990, the shires to the east and west of ‘Ucarro’ had 9.1 and 2.4% of agricultural land salt-affected, respectively. In both shires, a 0.7% increase in salt-affected land has occurred in the last 8 years.