Water Use and Productivity of Wheat under Five Irrigation Treatments

Water use and yield of wheat (Triticum aestivum L. em. Thell, cultivar Bluebird 2) were compared under five irrigation treatments using wheat planted in a silty clay loam soil in December and harvested in May. Water was applied at five rates from 11% more to 23% less than evapotranspiration (ET) from a weighing lysimeter planted to the same crop. Equal quantities of water per irrigation were applied at 76 mm in the pre-emergence and the first postemergence irrigations and 102 mm in later irrigations. For a 102-mm application, an irrigation was scheduled whenever cumulative ET from the lysimeter reached 91, 102, 112, 122, and 132 mm of water, since the last scheduled irrigation for the treatments. Total amounts of 485 to 688 mm of water were applied in five to seven irrigations. Soil water content to 1.5 m decreased progressively with water applications below ET. The soil water content was determined gravimetrically before each irrigation and water use determined from differences in soil water content between irrigations plus the amount of applied water. Seasonal water use decreased progressively from 677 to 540 mm as irrigation frequency decreased, although water use in all treatments was similar for the first 120 days. Yields decreased as water applications decreased, except between the treatments equal to ET and at 10% below ET. At the three driest treatments yield decreased proportionately with water use, indicating that water use efficiency was not increased by restricting water use below ET from adequately watered wheat. A relationship between grain yield (Y), in g/m², and water use (X), in mm water, was characterized by the expression: Y = −9548 + 46.42X − 0.06866X² + 0.00003390X³ (r = 0.98). Seed weight was unaffected by irrigation treatment. Cumulative total dry weight was similar on all treatments for the first 120 days. Plants headed 7 to 10 days earlier and matured 5 or 6 days earlier on the driest treatment than on the wettest treatments. Earlier maturity could have accounted for most of the differences in water use and total dry matter between treatments, although water stress was also important in the driest treatment. Plants produced more heads on the drier than on the wetter treatments and were shorter on the driest treatment. Data on the salt balance of the soil profile were inconclusive. The significance of these data to irrigation scheduling is discussed.