Irrigation for crops in a sub-humid environment

Summary The water use of two soybean cultivars (Bragg and Ruse) was measured for three seasons for a range of irrigation treatments. The seasonal totals of plant and soil evaporation ranged from 450 to 750 mm or from 36 to 64% of class A pan evaporation for the same period. Both cultivars extracted approximately 60% of the total extractable soil water in the top 1.2 m of soil before actual evaporation (Ea) dropped below potential evaporation (Eo). Up to this point the ratio between Ea and class A pan evaporation averaged 0.8. Ruse used water at a faster rate than Bragg but Ruse was not as effective in extracting the deep (below 1.0 m) soil water as Bragg. Water use efficiency (kg seed ha^–1 mm^–1 water) showed a small but general increase with decreasing irrigation water application. Runoff losses varied from zero for non-irrigated Ruse in 1977/78 to 352 mm for frequently-irrigated Bragg in 1976/77, generally increasing with the number of irrigations.