Expansion of the stress-day index for irrigation scheduling of grain sorghum

Abstract

The stress-day index is a concept whereby plant-water stress is characterized quantitatively. Its formulation consists of the product of a stress-day factor and a crop-susceptibility factor. The stress-day factor is an indication of the water deficit experienced by a crop. The crop-susceptibility factor is an indication of the tendency of the crop to be adversely affected by water deficit during a particular period of growth. The stress-day index is designed to correlate highly with crop yield. The stress-day index has been demonstrated to be an efficient approach for scheduling irrigation. An experiment was performed to expand the use of this approach to grain sorghum by determining the value of the crop-susceptibility factor for each of three stages of growth. The experiment also included an investigation of certain characterizations of the stress-day factor component of the stress-day index. Grain sorghum was grown in field lysimeters which provided complete control of the soil-water balance. A movable shelter, automatically actuated by rain fall, protected the lysimeters from rain. Different groups of plants were subjected to high water deficit during different growth stages. Each group of plants experienced approximately the same level of water deficit. Grain yields were determined after harvesting. Crop-susceptibility factors were calculated from the yields in the various treatments. These factors were determined to be 0.17 for the late vegetative to early reproductive growth stage, 0.34 for the boot to bloom stage, and 0.10 for the milk to soft dough stage. The following expressions were used in characterizing the stress-day factor: (1) Soil-water potential; (2) product of soil-water potential and maximum possible evapotranspiration; (3) early morning plant-water potential; (4) afternoon plant-water potential; and (5) average of early morning and previous afternoon plant-water potentials. Four different computational forms reflecting magnitude and duration of high water deficit were used with each expression within each period of growth. These computational forms were: (1) The sum of all of the daily expression values; (2) the sum of the excesses of the daily expression values above the values for a well-watered crop; (3) the sum of the excesses of the daily expression values above a critical level; and (4) the peak expression value. ...