Water use by annual crops. 1. Role of dry matter production

Abstract

In southern Australia, expanding dryland salinity is the result of increased deep drainage associated with widespread replacement of native perennial vegetation by annual agricultural crops and pastures. Although perennial pastures have been shown to assist in slowing salinisation, their adoption has been slow, and annual crops and pastures are likely to remain as the dominant land use for the foreseeable future. Therefore, understanding the water balance of annual crops and pastures, and how it can be manipulated, is important in trying to manage salinity. In this research we investigate the effect of varying levels of dry matter production on components of the water balance (soil evaporation, transpiration, soil water storage, and drainage) for annual crops at contrasting sites and soil types in south-western Australia. Dry matter production was controlled by fertiliser addition and crop rotation, and varied by a factor of up to 2, depending on seasonal conditions. Deep drainage was zero for most sites and years, but where it was greater than zero, there was no discernible effect due to production level. Out of a total of 14 site/year comparisons, the difference in soil water extraction associated with greater dry matter production averaged 5 mm, and was greater than 20 mm on only 1 occasion. However, high dry matter production was associated with greater transpiration, at the expense of soil evaporation. Manipulating dry matter production is unlikely to have a substantial effect on deep drainage and the expansion of dryland salinity in south-western Australia.