Evaluation of compensated heat-pulse velocity method to determine vine transpiration using combined measurements of eddy covariance system and microlysimeters

A field experiment was carried out with the objective to evaluate the compensated heat-pulse velocity (CHPV) method used to determine vine transpiration (Tsap). The performance of the CHPV method was evaluated using daily values of residual transpiration (Tr), obtained as the difference between actual evapotranspiration (ETa) and soil evaporation (Es) (Tr = ETa − Es) measured from an eddy covariance (EC) system and microlysimeters, respectively. Data used in this study were collected over a drip-irrigated Merlot vineyard trained on a vertical shoot positioned (VSP) system during three consecutive growing seasons (2006/2007, 2007/2008 and 2008/2009). Results showed that the best agreement between Tsap and Tr was obtained using correction coefficients for a wound size of 2.4 mm. The comparison between Tsap and Tr indicated that the index of agreement (d) was 0.97, and root mean square error (RMSE), mean absolute error (MAE) and mean bias error (MBE) were 0.22, 0.18 and −0.04 mm day−1, respectively. Also, the sensitivity analysis of fraction of wood (FM), fraction of water (FL) and factor of thermal properties of the woody matrix (k) suggested that the changes of ±30% have a little effect in the final estimation of daily vine transpiration with variations less than 12%. Finally, major disagreements between Tr and Tsap were observed on partially cloudy days where rapid changes (on 30 min time intervals) of solar radiation produced extreme values of volumetric sap flux density.