Determining Ideal Irrigation Termination Dates Under Deficit Irrigation Strategies

Irrigation termination is a very important, but complicated decision in cotton (Gossypium hirsutum L.) production due to complex interactions and uncertainties related to irrigation water use, cotton yield and fiber quality, water costs, and market factors. While an early termination of irrigation results in yield loss, late termination results in the wastage of valuable irrigation water, delays harvest and increases pest management costs. Implementing appropriate irrigation termination dates along with deficit irrigation strategies could not only ensure good cotton yield, but also increase irrigation water use efficiency (IWUE). In this study, a previously evaluated CROPGRO-Cotton module within the Decision Support System for Agrotechnology Transfer (DSSAT) Cropping System Model (CSM) was used. The model was evaluated using measured data from a cotton IWUE field experiment conducted at Halfway, TX during 2010 to 2013. The treatment factors in the field experiment included irrigation capacity (0, 3.2 and 6.4 mm/day) and irrigation application within three specific growth stage windows of pre-bloom, peak bloom, and post peak bloom. The evaluated model was used to simulate the long-term (1978-2016) effects of a combination of deficit/excess irrigation practices (Evapotranspiration (ET)-replacement of 55%, 70%, 85%, 100% and 115%) and irrigation termination dates (ranging from August 15^th to September 30^th with one-week intervals). Based on the simulated cotton yield, the optimum irrigation termination period was found to be the 1^th week of September for 115% ET-replacement, 2^nd week of September for 100% and 85% ET-replacement, and 3^rd week of September for 70% and 55% ET-replacement irrigation strategies. Based on the simulated IWUE, the optimum irrigation termination period was found to be the 1^th week of September for 115% ET-replacement, 2^nd week of September for 100% ET-replacement, and 3^rd week of September for 85%, 70% and 55% ET-replacement irrigation strategies.