Water flux below the root zone vs. irrigation amount in drip-irrigated corn

With increasing demands on water resources, greater efficiency is needed in irrigated agriculture. Internal drainage from the root zone is a loss that can be reduced or managed to improve irrigation efficiency. Our objective was to determine the relationship between seasonal water flux below the root zone (1.5 m) and irrigation amount with corn (Zea mays L.). Subsurface drip irrigation systems near Colby and Holcomb, KS, were used to supply water. Driplines were buried at a soil depth of 0.40 to 0.45 m, with a spacing of 1.5 m. The two soils are deep silt loams that formed in loess. Water flux at the 1.5-m soil depth was determined in four irrigation treatments during 1990 and 1991. Tensiometers were placed at soil depths of 1.4 and 1.7 m and at distances from the dripline of 0, 0.4, and 0.8 m. Water flux was calculated using predetermined hydraulic conductivity vs. matric potential (psi(m)) relationships, psi(m) data from tensiometers within the corn plots, and Darcg's equation of,vater now. irrigation was applied to four treatments such that irrigation plus rain equaled 125, 100, 75, and 50% of calculated corn evapotranspiration (ET). From a regression analysis relating integrated water flux below the root zone (1.5 m) and in-season irrigation, net upward water flux occurred with in-season irrigation < 296 mm (RMSE = 71 mm),,whereas net downward water flux occurred with irrigation > 296 mm. Compared with the 100% ET treatment (full irrigation), the 75% ET treatment had 76% of the in-season irrigation, 25% of the in-season water flux (net downward) below the root zone (1.5 m), and 93% of the corn groin yield. Near-maximum corn grain yields can be obtained with significant decreases in irrigation amount and internal drainage from the root zone compared with full irrigation.