COMPONENTS OF THE WATER-BALANCE ON DUPLEX SOILS IN WESTERN AUSTRALIA

The water balance of soils is of interest both in terms of crop productivity and of sustainability, because water that is not used by crops contributes not only to loss of yield but also to the processes of land degradation. Land clearance for agriculture in Western Australia has changed the hydrological balance resulting in greater drainage and increased secondary salinisation. The relative importance of components of the water balance changes with rainfall. In the drier, eastern wheatbelt, evapotranspiration dominates the balance and there is little drainage, except through narrow, preferred pathways. In contrast, in the wetter region of the Darling Peneplain up to 40% of rainfall may be lost by pathways other than evaporation. Movement of water at the interface of the sand and clay layers (throughflow) has been poorly quantified. The dominant direction of flow is vertical, but substantial lateral flow may occur during winter storms on sloping sites. For example, measurements at a site near Narrogin showed up to 15 mm of throughflow during the cropping season. Drainage using interceptor drains provides a practical means of removing the throughflow and reducing the incidence of waterlogging. Yields of wheat on duplex soils are poorly related to the amount of evapotranspiration. At low rainfall, the relationship is poor, because rainfall is rarely uniformly distributed throughout the season, and lack of water during grain-filling may depress yields. In high rainfall areas, there are few detailed studies of water use, but yields may be lower than anticipated because of waterlogging, low inputs of fertiliser, and disease.