Measuring vine 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 output of the sensors was then validated against transpiration of vines growing in the weighing lysimeter.
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. In addition, a mini-lysimeter (small grapevines growing in a pot setting on a scale) was also used to make a comparison between water use and sap flow measured with the sensors.
Grapevine water use measured with the weighing lysimeter is minimal during the evening and increases rapidly during the day with a maximum value near solar noon and then decreases as the sun sets in the afternoon. During the 2008 growing season maximum daily water use was in excess of 65 L day-1 (> 17 gallons) with maximum hourly water use of 8 L (~2.1 gallons). The maximum crop coefficient was greater than 1.35, which occurred the end of July. The daily patterns of sap flow velocity measured with the dual CHPM-HMR method mimicked the daily pattern of grapevine water use measured with the weighing lysimeter. Peak values of sap flow velocity converted to liters of water per vine per hour were similar to those measured by the lysimeter. The sap flow sensors responded quickly to changes in grapevine water use brought about by shading the vines for an hour and then removing the shade. The sap flow sensors also demonstrated the decrease in daily water use after irrigation was terminated for a period of 9 days and then showed a rapid increase in water use once irrigation resumed.
The results indicated that water use measured with the weighing lysimeter and with sap flow sensors were highly correlated with one another. The CHPM method was able to measure high flow rates while the HRM method was somewhat better at detecting low flow velocities. Once the cross-sectional area of the xylem active in transporting water has been determined, values of grapevine transpiration will be accurately determined with the two sap flow methods used in this study.