Determination of Grass Evapotranspiration Rates and Crop Coefficients Using Eddy Covariance Method in Eastern North Dakota

Accurate evapotranspiration (ET) estimation is essential to crop water management and irrigation scheduling. As a major agricultural area, North Dakota lacks studies on crop ET measurement for major crops. In this study, an eddy covariance system (EC) and two sets of soil moisture sensors were installed to measure cool season grass ET and estimate the crop coefficient (K-c) in eastern North Dakota from January 2011 to December 2013. The residual energy balance and Bowen ratio method were used to calculate the latent heat flux (LE) and ET values. Priestly-Taylor method also was used to estimate the missing values. Daily and monthly variability in fluxes and ET were quantitatively analyzed. For the study period, the soil moisture content for grass root zone has been continuously monitored and the relation between flux data, weather data, and soil moisture content were illustrated. The results showed that the growing season ET during 2011-2013 ranged from 560 mm to 773 mm from April to October, with the highest ET in July. The annual difference in ET was related to differences in weather and soil moisture conditions. The Kc for grass was developed as a ratio of actual ET measured by the EC, and the reference ET using ASCE-EWRI ET reference method. Seasonally, the highest ET and K-c were observed in July and April, respectively. The average K-c values for 2011, 2012 and 2013 were 0.97, 0.65 and 0.85, respectively. Therefore, 0.8 is recommended K-c value based on cool season grass as a reference crop for the study area during the growing season.