Crop water use and crop coefficients of sunflower in the U.S. Central Great Plains

Sunflower is a drought-tolerant crop commonly grown as a rainfed crop in semi-arid areas, but irrigation is required to produce maximum yield. The purpose of this study was to determine sunflower water requirements and crop coefficients to facilitate irrigation planning and precision scheduling. Sunflower crop water use (evapotranspiration, ETc) was measured by water balance in a 9-year field trial (2008–2016) in the west-central U.S. Great Plains. Seasonal ETc of the well-irrigated crops varied from 422 to 660 mm and averaged 523 mm. About 10 % of the seasonal ETc with surface drip irrigation was estimated to be evaporation from the wet soil surface following precipitation or irrigation. Seasonal ETc averaged 67 % of alfalfa (tall crop) reference evapotranspiration and 81 % of grass (short crop) reference evapotranspiration. Derived mid-season basal alfalfa reference crop coefficients, averaged 1.09 but varied among years from 0.94 to 1.32. Derived mid-season climate-adjusted basal grass reference crop coefficients averaged 1.23 and likewise varied among years. The variability was not correlated with several climatic or cropping parameters. The derived basal crop coefficients were linearly related to crop canopy ground cover which provided an excellent way to scale mid-season crop coefficients during both crop development and maturation stages. These multiyear results indicate that crop coefficients may vary among years.