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ARS Home » Plains Area » Bushland, Texas » Conservation and Production Research Laboratory » Soil and Water Management Research » Research » Publications at this Location » Publication #233073
Title: Evaluating eddy covariance cotton ET measurements in an advective environment with large weighing lysimeters

Author
item Chavez Eguez, Jose
item Howell, Terry
item Copeland, Karen

Submitted to: Irrigation Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/30/2009
Publication Date: 9/3/2009
Citation: Chavez Eguez, J.L., Howell, T.A., Copeland, K.S. 2009. Evaluating eddy covariance cotton ET measurements in an advective environment with large weighing lysimeters. Irrigation Science. 28(1):35-50.

Interpretive Summary: Evapotranspiration estimates using remote sensing methods are used to monitor crop water use, to schedule irrigation, to infer about soil water status, and hydrologic as well as climatologic processes. Eddy covariance (EC) systems are being used to assess the accuracy of remote sensing methods in mapping surface heat fluxes and evapotranspiration at different spatial scales. Therefore, the evaluation of the accuracy of EC evapotranspiration measurements is fundamental. In this study, two EC systems were evaluated with large monolithic weighing lysimeters, on irrigated cotton fields in the Texas High Plains, with data collected during the months of June and July, 2008. Results indicated that the EC system under predicted daily evapotranspiration. However, after adjustments were performed, using the preservation of energy concept and other suggested steps, evapotranspiration estimation error decrease to only -0.03 ± 0.5 mm/d (or -0.6 ± 10.2%). These errors are acceptable because they were similar in magnitude to measurement errors obtained with instruments that determine the different energy balance components. There are many potential causes of EC under estimation of evapotranspiration, often difficult to account for, thus it is recommended that the methodology proposed in this study be applied to adjust EC evapotranspiration measurements. The methodology outlined in this study worked well in the semi-arid and advective climate of the Southern High Plains

Technical Abstract: Eddy covariance (EC) systems are being used to assess the accuracy of remote sensing methods in mapping surface heat fluxes and evapotranspiration from local to regional scales; as well as in crop coefficients development. Therefore, the evaluation of the accuracy of EC heat flux measurements is fundamental. In this study, two EC systems (EC1 and EC2) were installed near large monolithic weighing lysimeters, on irrigated cotton fields in the Texas High Plains, during the months of June and July, 2008. Sensible (H) and latent (LE) heat flux evaluation with large lysimeters indicated that the EC systems greatly underestimated reference values with an average error of about 30% and a standard deviation of 30-60%. Most of the errors were contributed by nocturnal measurements when the atmospheric stability condition tended to be stable to neutral and turbulent eddies were small or non-existing. Energy balance (EB) closure was found to be 73.2 to 78.0% for daytime heat flux values. Thus, average daylight heat fluxes were adjusted for lack of EB closure using the Bowen ratio (BR)/preservation of energy principle; which improved the resulting EC heat flux agreement with lysimetric values. Further adjustments to EC-based ET included nighttime ET (composite) incorporation, and the use of ‘heat flux source area’ (footprint) functions to compensate ET when the footprint expanded beyond the crop field boundary. The footprint model assessed the extent (fetch in two dimensions) and magnitude of the fluxes being contributed from particular source areas upwind of the EC stations. As a result, EC2-based ET values remarkably matched lysimetric ET values, with a ‘mean bias error ± root mean square error’ of only -0.03 ± 0.5 mm/d (or -0.6 ± 10.2%), with a correlation slope of 0.99, intercept of -0.029 and R squared of 0.93. EC-based ET composite estimation errors were even lower for values greater than 3 mm/d. There are many potential causes of EC under estimation of H and LE fluxes, often difficult to account for, thus it is recommended that EB closure be forced following the BR method. The methodology outlined in this study seems to work well in the semi-arid and advective climate of the Southern High Plains.