Actual crop coefficients for cereal crops in Montana USA from eddy covariance observations

Accurate quantification and derivation of crop coefficients (Kc) are essential for sustainable water management, especially in semi-arid agroecosystems facing water scarcity exacerbated by climate change. With the goal of creating a foundational local crop coefficient resource, we apply the FAO’s Penman-Monteith model to estimate evapotranspiration (ET) - evaporation from soils and non-stomatal surfaces, and transpiration from plants - and use eddy covariance and micrometeorological data to model actual Kc (Kc act) for spring wheat, winter wheat, and barley in semiarid agricultural regions of Montana, USA where growth-stage based Kc act has been infrequently reported. We used piecewise linear regression to calculate Kc act during different stages of the growing season. Kc act during the development stage ranged from 0.48 to 0.88 for flood-irrigated barley and non-irrigated wheat, peaked at most sites during the mid-stage (ranging from 0.28 to 0.69 for pivot-irrigated spring wheat), and linearly increased and decreased during the early and late phases, respectively. Variability in derived Kc act was influenced by soil water content, vapor pressure deficit, and soil heat flux representing residual sensitivity to Kc act arising from atmospheric and soil water limitations even in irrigated systems. We anticipate that the Kc act values reported here will be useful and transferable for irrigation management in Montana and similar semi-arid climate regions.