Quantifying soybean evapotranspiration using an eddy covariance approach

Quantification of evapotranspiration (ETc) from crops is critical in irrigation scheduling in agriculture. In a pioneering study, in the Mississippi (MS) Delta region, we quantified ETc from soybean (Glycine max L.) using the eddy covariance (EC) approach (ETe). We also monitored ETc using a residual energy balance (EB) approach (ETb) and compared the fluxes. The unclosed energy fluxes in the EC were post-analysis closed using the Bowen ratio (BR) and latent heat (LH) methods. The measurements were conducted in a 35-ha clay soil planted to irrigated soybean in the lower MS Delta in 2016. The crop reached physiological maturity in 126 days after emergence (DAE). Maximum LAI was 5.7 and average grain yield was 4900 kg ha−1. The EC showed an energy balance closure of about 88% on a 30 min and 90% on a daily flux accumulation. The ETe was 18.2, 6.8, and 15.9% lower than ETb, and ETe corrected using BR (ETebr) and LH (ETele) approaches, respectively. Average soybean seasonal ETe, ETb, ETebr, and ETele were 422, 499, 451, and 490 mm, respectively. Seasonal reference-crop evapotranspiration for alfalfa (ETo) and grass (ETr) were 470 and 547 mm, respectively. Daily ETe, ETb, ETebr, ETele, ETo, and ETr averaged across the whole season were 4.4, 5.2, 4.7, 5.1, 4.9, and 5.7 mm, respectively. For scheduling irrigations, based on grass and alfalfa reference crop ET calculated from weather data, averages of the ETe, ETb, ETebr, and ETele daily estimates were used in deriving crop coefficients (Kc). The Kc for grass reference varied between 0.56 and 1.29 and for alfalfa reference varied between 0.56 and 1.02. The information developed will be useful for scheduling irrigations in the MS Delta region, and the methodology developed can be adapted for generating similar information elsewhere.