Cotton Yield Response to Variable Irrigation Capacity in the Texas High Plains

The objectives of this paper are to describe on a four-year cotton irrigation project conducted in the Texas High Plains and present cotton lint yield, fiber quality, and water use efficiency results from this field experiment. The treatment factors included in-season irrigation capacity (maximums of 0 in/d, 0.125 in/d, and 0.25 in/d) and irrigation application within a specific growth period.  Periods were determined by heat unit (hu) accumulation and were generally designated as early vegetative/juvenile (< 950 hu), reproductive (950-1350 hu) and maturation period (>1350 hu).  Combinations of these factor levels resulted in 27 irrigation regimes or treatments.  A 4-span LEPA pivot was used to irrigate 9.5 acres of this field experiment.  The pivot was modified so that each 8-row section (40-in circular rows) along the lateral length could automatically provide different irrigation amounts depending on the treatments being irrigated and pivot position.  Groups of four valves were actuated using signals from a controller with specific time sequences for each irrigation treatment and distance from the pivot point.  Inputs to the controller were pivot location (via GPS signal) and irrigation quantity (via application map) at each 8-row x 16-degree section for each irrigation sequence.  Test results to date were obtained from years of high rainfall in 2010 and low and ineffective rainfall in 2011 and 2012.  In all years, cotton yield and water productivity data indicated that building soil water in the profile, or irrigating in excess of the evapotranspiration rate of the cotton plants, reduced irrigation water value compared to applying irrigation later in the growing season.  This was attributed to water loss from excessive evaporation (high wind, low humidity) that often occurs in May and June on the Texas High Plains.