Estimating tree and vine evapotranspiration with emphasis on surface renewal

Advantages of various micrometeorological methods to estimate evapotranspiration are discussed in this paper with an emphasis on the 'new' surface renewal method. The surface renewal method for estimating sensible heat flux density in conjunction with net radiation and soil heat flux density measurements provides a relatively inexpensive technique to determine evapotranspiration from tree and vine crops. Fine-wire thermocouples are used to measure high-frequency temperature above and within canopies and structure functions are used to determine temperature ramp characteristics, which are used in a fundamental conservation of energy equation to estimate sensible heat flux density. Experiments were conducted during the summer of 1998 in a peach orchard near Atalaia, Portugal and in a grape vineyards in California and Italy. Surface renewal measurements taken at several heights in the canopies were compared with simultaneous sonic anemometer measurements of sensible heat flux density. The results indicate that the surface renewal technique provides good estimates of sensible heat flux density under most conditions. These results are encouraging because sonic anemometers are costly, they are damaged easily, and they give spurious results in some conditions. Using a thermocouple, the surface renewal technique offers an inexpensive alternative for estimating ET with good accuracy.