Investigating the Practicality of Deriving ET from Sonic Temperature.

We revisit the idea of using a modified eddy covariance (EC) approach that derives evapotranspiration (ET) or latent heat flux from sonic temperature flux and air temperature flux, thus eliminating the need for a hygrometer or water vapor analyzer.  Although this approach likely results in greater uncertainty of ET compared to traditional EC using a gas analyzer, it reduces costs associated with an analyzer while still providing a more direct measurement than a Penman-Monteith (PM) equation and may provide greater accuracy than a PM approach, at least during conditions when water stress is dynamic.  This approach has been suggested and studied before (e.g., Schotanus et al., 1983), however, widespread use has not been adopted, likely due to challenges in reducing errors and uncertainty in the measured difference between sonic temperature and air temperature.  Recent developments, however, may provide additional insight for making the approach more feasible.  These developments include using a more precise calculation, implementation of appropriate corrections, and improvements in sonic temperature accuracy.  In this study, a colocated and synchronized open-path water vapor analyzer provides a means to closely compare the approach with traditional EC.