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

Abstract

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.