Accuracy, Uncertainty, and Limitations of Eddy Covariance for Measuring Evapotranspiration.

Eddy covariance (EC) has become a standard method for measuring landscape-to-atmosphere fluxes of gases and energy across a variety of ecosystems. During daytime, when the atmosphere just above a uniform landscape is well-mixed and relatively unchanging (near steady-state), EC fluxes will almost always be extremely reliable and trustworthy, which makes EC a nearly ideal method for measuring evapotranspiration (ET) from agricultural landscapes. But successfully applying the EC methodology does require particular care and some knowledge of physical and atmospheric fundamentals.  In the broadest terms the EC method measures fluxes [ET] by time-averaging the product (covariance) of fluctuations of gas [vapor] density and vertical wind speed above the surface. The accuracy of EC measurements are affected by many factors including surface conditions, fetch, atmospheric conditions, sensor design and configuration, data processing algorithms, and the complications of co-occurring heat and mass fluxes. Furthermore, the inherent lack of energy balance closure continues to be a source of some concern about possible biases in the EC methodology. This presentation gives an overview of EC theory applied to estimating ET and highlights the major factors that affect ET measurement accuracy.  Sources of ET measurement uncertainties will also be discussed and recent developments in EC science and instrumentation and energy balance closure will be summarized.