Crop water use efficiency from eddy covariance methods in cold water-limited regions.

• Observed crop water use efficiency(WUE) in Canadian prairies with eddy covariance. • WUE varies between crop type, soil, and weather and should not be considered static. • Eddy covariance reported lower and less variable WUE than water balance approaches. • Environmental factors that increase evaporative demand decrease WUE. Crop–water interactions define productivity in water-limited dryland agricultural production systems in cold regions. Despite the agronomic and economic importance of this relationship there are challenges in quantifying crop water use efficiency (WUE). To understand dynamics driving crop water use and agricultural productivity in these environments, observations of evapotranspiration, carbon assimilation, meteorology, and crop growth were collected over 17 site-years at 5 agricultural sites in the sub-humid continental Canadian Prairies. Eddy-covariance (EC) derived water and carbon fluxes provided a means to comprehensively assess the WUE of current agricultural practices by both physiological (WUE P : g C kg−1 H 2 O) and agronomic (WUE Y): kg yield mm H 2 O−1 hectare−1) approaches. Mean field scale WUE Y for grain yields were 10.4 (Barley), 10.2 (Wheat), 6.0 (Canola), 19.3 (Peas), 12.2 (Lentils) and for silage/forage crops were 23.0 (Barley), 11.9 (Forage), and 20.7 (Corn) (kg yield mm H 2 O−1 hectare−1). An assessment of environmental factors and their covariance with WUE , utilising a conditional inference tree approach, demonstrated that WUE decreased when crops were under greater evapotranspiration demands. EC-based areal WUE approaches, measuring fluxes over footprints of hundreds of square metres, were compared with more commonly reported point-scale water balance residual approaches (WUE WB) and demonstrated consistently smaller magnitudes. WUE WB was greater than EC-estimated WUE Y by an average of 52% and 65% for grain and forage/silage crops respectively. WUE WB also had greater variability than EC estimates, with standard deviations 188% and 128% greater than Barley and Wheat crops, respectively. This comparison highlights the scale dependency of WUE estimation methods, demonstrates considerable uncertainty in point scale water balance approaches due to spatial variability in crop–water interactions, and shows how this variability can be accounted for by EC observations. This improves the understanding of WUE and quantifies its variability in cold continental water-limited climates and provides a means to diagnose improved agricultural water management. [ABSTRACT FROM AUTHOR]