If the sonic sensor in a covariance system is not well leveled, the covariance calculations will produce errors. One may rotate the data to obtain the accurate ET data. There are different rotation methods. The traditional method is to set the vertical wind velocity (W) to be zero and the mean wind direction to be perpendicular to the W direction (Tanner and Thurtell, 1969; Kaimal and Finnigan, 1994). Other rotation methods were also developed (Lee et al., 2004). The errors (unrotated flux) depend on the instrument tilting level. The larger the tilting angle, the larger the errors (unrotated flux). Usually, the error is about 0-25% (http://www.licor.com/env/Products/GasAnalyzers/eddyPresentation/EC_master_Presentation _files/_frame.htm).
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