Improving crop water use determination using adjusted eddy covariance heat fluxes

Abstract

Eddy covariance (EC) systems are being used to measure sensible heat (H) and latent heat (LE) fluxes in order to determine crop water use or crop evapotranspiration (ETc). However, EC derived H and LE fluxes must be adjusted because EC systems systematically tend to underestimate actual H and LE heat fluxes. Thus, the energy balance does not close well generally for EC heat fluxes. A standard procedure for adjusting EC fluxes takes environmental conditions into consideration. This procedure allows for an improved determination of ETc values on an hourly and daily basis. The objective of this study was to evaluate different adjustment methods and determine which one or combination of adjustment methods improved performance. For this purpose, two EC systems were installed near two large monolithic weighing lysimeters on irrigated cotton fields at the USDA-ARS Conservation and Production Research Laboratory at Bushland, Texas in the Texas High Plains during the months of July and August of 2008. A total of eight days (four in July and four in August) of EC data from two adjacent fields were post-processed and results were compared with the lysimetric ETc. The evaluation included an analysis of the EC energy balance (EB) closure and residuals. Results indicated that the adjusted LE heat fluxes (converted to an equivalent amount of evapotranspirated water depth) underestimated the measured ETc with an average error of 19-21% compared with 25-27% for the LE fluxes that were not adjusted. The residual errors occurred mostly during nocturnal measurements. The EB closure for the adjusted daytime fluxes was 88-97% which was an improvement over the 83-89% closure before adjustments. The EC frequency response adjustment had the largest impact in improving the EC-based ET values. This adjustment increased the LE fluxes by an average of 5.0- 5.5%, and thus it was the most significant adjustment. Therefore we recommend that the combined adjustment method described be consistently applied when using EC to properly measure ETc using EC systems for irrigated cotton in the Texas High Plains.