Water flux below the root zone vs. drip-line spacing in drip-irrigated corn

Use of microirrigation (drip) is increasing, prompted by factors such as a greater ability to control losses of water and nutrients from the root zone. The cost of drip can be reduced by using wider drip-line spacings. Our objective was to evaluate water flux below the root zone (1.5-m soil depth) with a subsurface drip irrigation system having drip-line spacings of 1.5, 2.3, and 3.1 m near Colby, KS. The soil is a deep silt loam that formed in loess. The crop was corn (Zea mays L.), planted in rows spared 0.76 m apart. Water flux at the 1.5-m soil depth was determined in five treatments (Trt.): 1.5-m spacing, full irrigation; 2.3-m spacing, 67% of full irrigation; 2.3-m spacing, full irrigation; 3.1-m spacing, 50% of full irrigation; and 3.1-m spacing, full irrigation. With full irrigation, irrigation plus effective rain equaled calculated corn evapotranspiration (ET). Tensiometers were placed below the drip line and at increments of 0.4 m from the drip line at soil depths of 1.4 and 1.7 m. Water flux was calculated by using a hydraulic conductivity (K) vs, matric potential (Psi(m)) relationship, Psi(m) data from tensiometers within the corn plots, and Darcy's equation of water flow. In-season water fluxes below the root zone (1.5 m) were 51, 118, and 124 mm at drip-line spacings of 1.5, 2.3, and 3.1 m, respectively. In 1990, corn grain yield was not significantly affected by spacing. In 1991, with drier initial soil-water conditions, corn yielded significantly less grain at spacings of 2.3 m (11.5 Mg ha(-1)) and 3.1 m (10.7 Mg ha(-1)) than at a drip-line sparing of 1.5 m (13.1 Mg ha(-1)). If spacing between drip lines is increased beyond 1.5 m in the silt loam soils of western Kansas, there would be an associated increase in internal drainage from the root zone and decrease in corn yields.