Crop Evapotranspiration and Crop Coefficients - Sources of Uncertainty.

Accurate quantification of crop evapotranspiration (ET) is critical for water management. However, existing mass-balance field approaches to the quantification of ET are based on assumptions about water flux at the lower boundary of the soil profile. In many cases, these assumptions have not been verified. Agricultural simulation models such as Root Zone Water Quality Model (RZWQM2) are commonly used to simulate the water balance for different soil and climate conditions. However, in some cases, there is an uncertainty in simulated soil water content which could deviate from measurements. The objective of this project is to enhance the understanding of quantifying actual evapotranspiration and to evaluate the performance of the RZWQM2 in simulating soil water dynamics and crop evapotranspiration. An experiment was conducted at Spindletop Research Farm, Lexington, Ky. Soybean and corn were grown in 2015 and 2016, respectively. ET was quantified based on three approaches which are i) water balance approach (WB), ii) Penman-Monteith approach (FAO), and iii) simulated by RZWQM. Crop coefficient (Kc) was quantified based on three methods i) measured ET and reference crops ET, ii) simulated ET and reference crops ET, and FAO method. RMSDs of daily ETa were less than 1 mm for both crops, with a slight overprediction by the RZWQM model and FAO. This result indicated that the model predicted ETa reasonably well for the entire seasons. The Kc values obtained using simulated ET were closer to FAO values and slightly differed from the Kc values based on measured ET with RMSDs between 0.12 and 0.26 for both crops.