Surface renewal estimates of evapotranspiration. Tall canopies.

Sensible heat flux density (H-SR) was determined over grapevine and avocado tree canopies using the surface renewal (SR) method that was first proposed by Paw U and Brunet (1991). The method employs high frequency temperature measurements above canopies to calculate H values using a conservation of energy equation. The H-SR estimates increased with measurement height above the canopies, and a calibration factor (a) was needed to improve H estimates. This a is believed to result from unequal heating of the mean air volume under the measurement height. The grapevines have a sparse canopy, whereas the avocado orchard has a dense canopy. This may explain the difference in observed a values. However, more research is needed to better understand a. Evapotranspiration (lE(SR)) was calculated using net radiation and soil heat flux density and H-SR in the energy balance equation. An independent measure of H, obtained from a sonic anemometer, was used to calculate evapotranspiration (lE) for comparison with lE(SR) values. High correlations were observed between lE(SR) and lE for both crops. When the a values were used to determine H-SR and a regression of lE(SR) versus lE was computed, the coefficient of determination values were R-2 = 0.96 for avocado trees and R-2 = 0.93 for grapevines.