SCHEDULING IRRIGATIONS FOR SOYBEANS WITH THE CROP WATER-STRESS INDEX (CWSI)

A Crop Water Stress Index (CWSI) has been related to water use and plant water-stress parameters, but irrigation scheduling based on CWSI values has only been reported for corn and wheat. The objectives of this study were to evaluate irrigation scheduling of soybean (Glycine max L. Merrill) with CWSI as computed from measurements of infrared canopy temperature, air temperature, and vapor-pressure deficit, and to determine if amount of water applied per irrigation influenced water use and yield obtained from irrigation scheduling by CWSI. Soybeans were grown in field plots with drip irrigation (25 mm water per irrigation) and under a rainout shelter with flood irrigation (25 or 51 mm water per irrigation). Irrigations were initiated when CWSI reached threshold values of 0.1, 0.2, 0.4, or 0.6. Analysis of the data showed the CWSI baseline equation to be incorrect. A new baseline was computed and reanalysis of the CWSI data showed true irrigation thresholds had been 0.2, 0.3, 0.4, and 0.5. These four threshold values resulted in total irrigation amounts of 181, 180, 174, and 145 mm applied to the drip-irrigated plots, respectively. Under the rainout shelter, total irrigation amounts of 347, 271, 195, and 195 mm when 25 mm was applied per irrigation, and 406, 356, 356, and 305 mm when 51 mm was applied per irrigation, resulted from the 0.2, 0.3, 0.4, and 0.5 CWSI thresholds, respectively. Respective yields for these plots were 2656, 2566, 2430, and 2189 kg ha-1 for the drip-irrigated plots, 3375, 2826, 2435, and 2365 kg ha-1 for the rainout-shelter plots with 25 mm per irrigation, and 3575, 3551, 3110, and 2108 kg ha-1 for the rainout-shelter plots with 51 mm per irrigation. Under deficit-irrigation conditions the relationship between CWSI, soil water content, and leaf water-potential appears to change. Knowing how much water to apply per irrigation is important information for effectively using CWSI to schedule irrigations in soybeans. Cloudy sky conditions do not occur with sufficient frequency in the central Great Plains to inhibit the timely use of the infrared thermometer for irrigation scheduling. © 1990.