Evapotranspiration and Water Stress Coefficient for Deficit-Irrigated Maize
Thomas J. Trout, Kendall C. DeJonge
Abstract
Crop evapotranspiration under deficit soil water conditions must be quantified to accurately manage deficit irrigation and crop water stress and achieve targeted water savings. A stress coefficient is commonly used to quantify the effect of inadequate soil water on crop evapotranspiration. A 6-year deficit irrigation field trial of maize in northeastern Colorado was used to derive the stress coefficient for maize. Results showed that crop evapotranspiration was affected by both the effect of current soil water deficit on water uptake and stomatal resistance and the impact of prior water stress on plant growth. Measured evapotranspiration was less than potential crop evapotranspiration when the soil water deficit exceeded 25% of the total plant available water. A curvilinear relationship modeled the measured stress coefficient better than the commonly used linear relationship. Prior water stress resulted in a reduction in canopy ground cover, which was linearly related to the basal crop coefficient. These combined effects, when incorporated into the crop coefficient times reference evapotranspiration model, were able to accurately estimate crop evapotranspiration with deficit irrigation.