Water use efficiency and water use of Mediterranean annual pastures in southern Australia

There is a perception in the farming and research communities that annual p astures have low productivity and water use, and contribute disproportionat ely to problems of rising watertables and dryland salinity. Our aim was to determine potential pasture production in relation to water use and the inf luence of management factors on this relationship. Experiments were initiat ed at 4 locations along a gradient of 300-1100 mm annual rainfall across th e Western Australian agricultural zone. At each site a high input treatment was compared with a low input control. There was a strong linear relations hip between water use and pasture production up to 440 mm of growing-season water use. After 30 mm of water use the potential pasture production was 3 0 kg/ha.mm. An upper limit to pasture production may be reached at about 12 000 kg/ha in this environment due to rainfall distribution patterns and so il water holding capacity in the root-zone. Although pasture production was increased by as much as 3500 kg/ha, water use was generally similar or onl y slightly more for high input compared with control plots. The marginally higher water use by the high input pastures resulted in an extra 18 mm of w ater extracted from the subsoil at one location by the end of the third sea son. A drier subsoil may provide a buffer for storing excess rainfall and r educe deep drainage. Estimated drainage was small at low rainfall sites so even marginal increases in water use by highly productive annual pastures c ould play a significant role in reducing water loss to deep drainage and mi tigating water-table rise and secondary salinisation in low rainfall region s. Management practices aimed at promoting early growth and adequate leaf a rea should maximise water use, water use efficiency, and yield. The linear relationship defining potential pasture production provides a useful benchm ark to farmers.