Estimating sensible and latent heat flux densities from grapevine canopies using surface renewal

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