(e.g., CSAT3, Campbell Sci., Inc., Logan, Utah) to measure ET, which can be expressed as latent heat of vaporization, LE. In addition, the system can measure virtual air temperature and H, where H is sensible heat to the atmosphere (Wang et al., 2007). However, this technology has energy closure problems in cases where net radiation (Rn) does not equal LE + H + G, where G is sensible heat to the soil. For example, the average energy closure rate at the Fluxnet (22 towers) is 75%, and the rate is sometimes below 40% (Wilson et al., 2002). (FLUXNET is a global network of micrometeorological tower sites that use eddy covariance methods to measure the exchanges of carbon dioxide, water vapor, and energy between terrestrial ecosystems and the atmosphere.) This problem can be caused by low wind speed, stable conditions or horizontal flux. The apparent underestimate of LE may also relate to canopy roughness. In addition, wind sensor leveling, air humidity, and footprint can affect the ET measurement accuracy.
This commentary discusses how to check ET measurement accuracy and how to measure accurate ET using eddy covariance technology.
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