ABSTRACT

Cotton production might benefit from planned, non-uniform distribution of irrigation as a function of soil water holding capacity (SWHC) and topography leading to better utilization of both rainfall and irrigation in water short regions of the Texas High Plains. The four most outer spans of an 8-tower center pivot system were modified to deliver variable-rate (VR) irrigation within cells no larger than 0.1 acre. LEPA applicators were modified to provide relative flow rates of 2x, 3x, and 4x allowing stepwise increases in irrigation discharge of 20% of a base irrigation quantity. A control system was installed to actuate solenoid valves relative to field location, thereby controlling irrigation quantities at specific sites.

Field experiments were conducted in 2001 and 2002 to evaluate equipment and to document potential advantages of VR irrigation of cotton over standard practices. Alternating strips of cotton, 20 to 22 rows wide, were irrigated by either variable-rate (VR) or uniform-rate (UR) irrigation. In 2001, the VR irrigation strategy attempted to level lint yields by reducing irrigation in areas of high SWHC and adding water to areas of low SWHC. Management zones were based on soil texture and slope in a 12-acre area. In 2002, irrigation quantities were increased above the base rates in areas thought to be "more productive". Soil electrical conductivity (EC) was used to determine the management zones on a 15.4-acre test area for site-specific irrigation.

Final hydraulic evaluations of the VR irrigation system following construction in 2001 and modification in 2002 resulted in actual applicator flow rates within 5% of achievable flow rates. Errors in initial pivot positioning were documented. Based on the preliminary comparisons and the criteria used to designate management zones, VR irrigation of cotton produced no significant increase in total lint yield or total irrigation water use efficiency (WUE) over uniform LEPA application in 2001 or increases in WUE in 2002. Using soil EC as the criterion to establish management zones for VR irrigation resulted in lint yield increases of 2 to 4 % over uniform irrigation, but at the cost of additional water inputs.