Measuring Grapevine Transpiration Using Sap Flow Sensors: Validation / Calibration of a New Sap Flow Technique on Large Grapevines Growing in a Weighing Lysimeter

Weighing lysimeters are the standard for crop evapotranspiration (ETc) measurements. A large weighing lysimeter at the Kearney Agricultural Center has been successfully used since 1987 to measure water use of Thompson Seedless grapevines during vineyard establishment and once the vines were mature. While weighing lysimeters will provide a direct measure of grapevine water use, they are expensive to build and much time is needed to ensure their measurements are accurate. An alternative, allowing the accurate measurement of many vines at one time and highly portable, would be useful in viticulture. One such technique would be the use of sap flow sensors, which have been used to measure transpiration of young and mature grapevines. In this study we further developed and implemented a newly modified sap flow technique capable of precisely measuring both high and low rates of grapevine transpiration. The compensation heat pulse method (CHPM), which has been shown to work well under high flow rate conditions, and the heat ratio method (HRM), shown to work well under low and reverse flow conditions, were used to measure sap flow in this study. These sensors were installed in the trunks of the vines in the lysimeter and on several vines growing outside the lysimeter. Sap flow velocity was converted into water volume per hour by obtaining an estimate of the cross-sectional area of the trunk?s xylem active in the transport of water. Heat pulse techniques closely tracked diurnal grapevine water use determined through lysimetry in both the 2008 and 2009 growing season. This was true even at very high flow rates (> 6 Lvine-1 h-1) under which these techniques had not been previously tested in grapevines where measurements are more prone to error and errors would have the greatest effect on cumulative measurements. Volumetric water use determined with heat pulse techniques was highly correlated to hourly lysimeter water use both years, but the nature of the relationship was inconsistent from one year to the next (y = 0.45 + 0.33x; R2 = 0.92 in 2008, y = -1.00 + 1.47x; R2 = 0.95 in 2009). Similar results were obtained when comparing grapevine water use determined from heat pulse techniques to meteorological estimates for field-grown vines in Napa Valley and Davis and to bench scale lysimetry of two year-old potted vines grown in a greenhouse. The inconsistency in the regression coefficients obtained from each of these data sets was likely due to radial and circumferential variation in sap flow through the vines? trunks. However, the robust nature of all of the correlations demonstrates that heat pulse techniques can be used to measure grapevine water use after an in situ calibration. Conversely, the data from the sensors could be utilized as a measure of vine water status, similar to that of a pressure chamber used to measure leaf water potential. Once a critical value had been measured, such a baseline, maximum daily heat pulse velocity (or relative heat pulse velocity), one would schedule an irrigation event. Such an example is given in Figure 2, where daily heat pulse velocity decreases due to the termination of irrigation. However, like data from a pressure chamber, this technique would not determine how much water to apply.