Measuring tree and vine ET with eddy covariance

The eddy covariance method is a viable method to measure evapotranspiration from tree and vine crops. However, one must be careful to measure properly the energy balance components and to apply corrections to the heat flux data to obtain good results. Based on several years of experience with energy balance measurements, we have found that failure to achieve the energy closure is often attributable to inaccurate soil heat flux density data. In addition, interference by wind blowing past the mounting tower can lead to spurious results. Although the benefit was small in our experiments, applying the corrections for air temperature and density changes (WPL) are recommended to eliminate errors in sensible and latent heat flux density. This is especially true for high elevations and for locations with extreme temperatures. We found that locating the net radiometer and the eddy covariance instruments above or between the rows seems to have little influence on the measurement as long as the sensors are not too close to the canopy elements indicating that the 'footprints' were not an important factor. In this paper, these aspects of measuring ET with the eddy covariance method and some results will be discussed.