Soil Water DepletionYield Relationships of Irrigated Sorghum, Wheat, and Soybeans

A PPROXIMATELY 4 billion m3 A of groundwater is pumped annually to irrigate sorghum, wheat and soybeans grown in the fine-textured soils of the Southern High Plains. Declining groundwater storage and the high costs of pump-ing necessitate that water be applied only when it is needed to prevent appreciable yield reductions. Rela-tionships between soil water deple-tion and grain yields provide a rational basis for scheduling irrigations based on soil water in J:he major root zone. Relationships between soil water depletion on Pullman clay loam and yields of grain sorghum, winter wheat and soybeans were derived from 12-crop years of irrigation water manage-ment research conducted at the USDA Southwestern Great Plains Research Center, Bushland, Texas. Significant curvilinear relationships, which were fit by the general quad-ratic equation, were obtained in all years. Measured soil water deple-tion in the 0- to 120-cm depth accounted for 57 to 99 percent of the treatment yield variability within years. Depletion of soil water in the 0-to 120-cm depth to -15 bars potential resulted in best estimate relative yields of 73 percent for adequately irri-gated grain sorghum, 80 percent for winter wheat, and 47 percent for soybeans. The shape of the depletion-relative yield curves in the drierthan-normal seasons were similar for the three crops. However, in the 20 to 70 percent relative yield range, soil water depletion in the 0- to 120-cm profile was approximately 2.7 cm greater for wheat and sorghum than for soybeans. The lowest soil water contents measured in the 0-to 120-cm profile under prolonged stress were 1.3, 4.6, and 5.6 cm below -15 bars potential for soy-beans, grain sorghum and winter wheat, respectively. Based on depletion to below -15 bars, plant avail-able water exceeded the -1/3 to -15 bar range by 8.3 percent for soybeans to 35.7 percent for winter wheat. The relationship between soil water depletion and relative yields of grain sorghum obtained from a graded furrow study was similar to relation-ships previously obtained from level border studies. Results indicate that data obtained from level border plots can be applied directly to graded furrow irrigations on simi-lar soils in the Southern High Plains.