In contrast to the energy balance, mass transfer and combination methods (Chapter 10) that utilise data from automatic weather stations or manually read meteorological instruments, the eddy flux or eddy covariance method (Chapter 10) requires dedicated sensors. Eddy flux calculations require data on vertical wind velocity and humidity sampled at sub-second (i.e. Hertz) frequencies. These data can be combined to give a direct measurement of the evapotranspiration. A three-dimensional sonic anemometer can be used to sample the wind velocities (Fig. 4.6), while a closed-path gas analyser or open-path krypton hygrometer (Fig. 4.7) can be used to measure humidity. A network of over 400 towers supporting these sensors has been deployed across the globe (Baldocchi et al., 2001), including 16 towers within the UK.
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