Eddy covariance quantification of soybean (Glycine max L.,) crop coefficients in a farmer’s field in a humid climate

For sustainable irrigated agriculture, scheduling irrigations based on accurate estimates of crop water requirements (ETc, crop evapotranspiration) are critical. ETc was estimated as a product of a reference crop evapotranspiration computed from weather data and a crop coefficient (Kc) in weather-based irrigation scheduling. In this investigation, an eddy covariance (EC) method was used for quantifying soybean (cv. Asgro 46X4) Kc in a farmer’s field under a humid climate. ETc quantified using the EC method was used for developing Kc for alfalfa (Kcr) and grass (Kco) reference crops computed from measured weather data. Experiments were conducted during three crop seasons (2017–2019) in a 500-ha furrow-irrigated soybean field—planted in silt loam soil in late April to early May and harvested in September. Harvested soybean yields were 4771, 5783, and 4909 kg ha−1, consuming 584, 640, and 593 mm ETc (average 605 mm), respectively, in 2017, 2018, and 2019. Monthly averaged daily ETc across the crop seasons varied between 2.1 mm in May 2019 to 6.2 mm in June 2018. Seasonally averaged daily ETc across the three crop seasons varied between 4.3 and 5.2 mm with an average of 4.8 mm. Across the crop seasons, ETc was 22% less and 2% greater than computed grass (ETo) and alfalfa (ETr) reference crop evapotranspiration. Monthly averaged daily Kco varied between 0.79 and 1.18, and Kcr ranged between 0.65 and 0.97. The Kc established can help develop soybean irrigation schedules, across climates and soils, based on ETo or ETr computed from real-time weather data.