Water use of wheat, barley, canola, and lucerne in the high rainfall zone of south-western Australia

Water use of wheat ( Triticum aestivum L.), barley ( Hordeum vulgare L.), canola ( Brassica napus L.), and lucerne ( Medicago sativa L.) was measured on a duplex soil in the high rainfall zone (HRZ) of south-western Australia from 2001 to 2003. Rainfall exceeded evapotranspiration in all years, resulting in transient perched watertables, subsurface waterlogging in 2002 and 2003, and loss of water by deep drainage and lateral flow in all years. There was no significant difference in water use among wheat, barley, and canola. Lucerne used water at a similar rate to annual crops during the winter and spring, but continued to extract 80-100 mm more water than the annual crops over the summer and autumn fallow period. This resulted in about 50 mm less drainage past the root-zone than for annual crops in the second and third years after the establishment of the lucerne. Crop water use was fully met by rainfall from sowing to anthesis and a significant amount of water (120-220 mm) was used during the post-anthesis period, resulting in a ratio of pre- to post-anthesis water use (ET a : ET pa ) of 1 : 1 to 2 : 1. These ratios were lower than the indicative value of 2 : 1 for limited water supply for grain filling. High water use during the post-anthesis period was attributed to high available soil water at anthesis, a large rooting depth (≥1.4 m), a high proportion (15%) of roots in the clay subsoil, and regular rainfall during grain filling. The pattern of seasonal water use by crops suggested that high dry matter at anthesis did not prematurely exhaust soil water for grain filling and that it is unlikely to affect dry matter accumulation during grain filling and final grain yield under these conditions.