Quantifying crop coefficients for a corn (Zea mays L.) and soybean (Glycine max L.) sequence

The most critical management practice for maximizing crop production is applying irrigation at the right time and in the right amount, based on crop-consumptive water use (evapotranspiration, ETc) that is not met by natural precipitation. Because ETc is difficult to measure directly in field conditions, it is commonly estimated by first calculating evapotranspiration for a standardized reference crop - typically grass (ETo) or alfalfa (ETr) -and then adjusting these values using crop-specific coefficients (Kco and Kcr). Accurate crop coefficients tailored to local soil, crop, and climate conditions are essential for sustainable irrigation scheduling. We developed Kco and Kcr values using eddy covariance–derived ETc measurements for corn (Zea mays L.) and soybean (Glycine max L.) grown in rotation from 2017 to 2020 in a subtropical humid climate. The crops were managed under non-limiting water and nitrogen conditions and were free from pest, disease, and weed pressures, ensuring optimal growth and allowing precise quantification of ETc for coefficient development following the standardized FAO (Food and Agricultural Organization) procedure for developing universally applicable crop coefficients. For corn, Kco ranged from 0.81 at emergence (VE) to 1.22 at tasseling (VT) and declined to 0.83 at physiological maturity (R6). Corresponding Kcr values were 0.60 at VE, 1.01 at VT, and 0.61 at R6. For soybean, Kco and Kcr were 0.69 and 0.64 at the vegetative (VE) stage, increased to 1.18 and 1.07 at full pod (R4), and declined to 0.92 and 0.79 at full maturity (R8). Across seasons, daily ETc ranged from 4.1 to 4.4 mm for corn and 4.4 to 5.5 mm for soybean. These crop coefficients provide a robust foundation for irrigation scheduling in corn–soybean rotation systems, supporting improved water-use efficiency. Additional research is warranted to quantify ETc and refine crop coefficients in regions with contrasting soils and climatic conditions. • Kc for corn and soybean in rotation quantified using the Eddy Covariance technique • Kc for alfalfa and grass reference crop derived for crop water requirements • Kco for corn in the soybean-corn rotation ranged between 0.81 and 1.23 • Kcr for corn in the Soybean-corn rotation ranged between 0.60 and 1.01 • Kco for soybean in the corn-soybean rotation ranged from 0.69 to 1.18 • Kcr for soybean in the corn-soybean rotation ranged from 0.64 to 1.07