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

Accurate quantification and derivation of crop coefficients (K c ) 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 K c (K c act ) for spring wheat, winter wheat, and barley in semiarid agricultural regions of Montana, USA where growth-stage based K c act has been infrequently reported. We used piecewise linear regression to calculate K c act during different stages of the growing season. K c 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 K c act was influenced by soil water content, vapor pressure deficit, and soil heat flux representing residual sensitivity to K c act arising from atmospheric and soil water limitations even in irrigated systems. We anticipate that the K c act values reported here will be useful and transferable for irrigation management in Montana and similar semi-arid climate regions. • Crop coefficients quantified for multiple Montana small grain fields. • Piecewise linear regression identified crop coefficient stages. • Actual crop coefficients (K c act ) ranged from 0.16 to 0.88 across crops and stages. • Hydrological variables impacted K c act , suggesting water stress in irrigated fields.