Estimating sensible and latent heat flux densities from grapevine canopiesusing surface renewal

Fine-wire thermocouples were used to measure high-frequency temperature abo ve and within canopies and structure functions were employed to determine t emperature ramp characteristics, which were used in a fundamental conservat ion of energy equation to estimate sensible heat flux density. Earlier expe riments over dense, tall, and short canopies demonstrated that the surface renewal method works, but requires a correction for uneven heating (e,g. al pha=0.5 for tall, and alpha=1.0 for short canopies). For sparse canopies, t he ct calibration factor was unknown. Experiments were conducted in grape v ineyards in California and Italy to determine whether the surface renewal m ethod works in a sparse canopy and to determine if calibration is necessary . Surface renewal data were collected at several heights in the canopies an d these were compared with simultaneous 1-D sonic anemometer measurements, The results indicated that the surface renewal technique provides good esti mates of sensible heat Aux density under all stability conditions without t he need for calibration when the data are measured at about 90% of the cano py height. The values were generally within ca, 45 Wm(-2) of what was measu red with a sonic anemometer. Separating the canopy into two layers provided even more accurate estimates of sensible heat flux density without the nee d for calibration The best results were obtained when the lower layer was b elow the bottom of the vegetation and the upper layer included the vegetati on. When combined with energy balance measurements of net radiation and soi l heat flux density, using a thermocouple and the surface renewal technique offers an inexpensive alternative for estimating evapotranspiration with g ood accuracy. (C) 2000 Elsevier Science B.V. All rights reserved.