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. 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) during the 2009 growing season. The maximum crop coefficient was greater than 1.3 and at harvest (end of August) averaged 1.1. The hourly patterns of grapevine water use measured across the season with the dual CHPM-HRM method were highly correlated (R2 > 0.95) with those obtained by the lysimeter. Sap flow sensors were also installed in Cabernet Sauvignon grapevines growing in Napa Valley (UC Oakville Research Station) and Chardonnay grapevines growing in Davis (a Department of Viticulture and Enology?s vineyard west of campus.) A comparison of the diurnal heat pulse velocity at the Parlier, Davis and Oakville sites were compared with one another on two selected dates. The diurnal heat pulse velocity closely mimicked that of evaporative demand throughout the day on June 14th at all three locations. The diurnal pattern of heat pulse velocity at the Parlier site again was maximized at solar noon (greatest evaporative demand) while those at Oakville had higher values later in the afternoon on August 20th. The diurnal pattern of heat pulse velocity at the Davis site was maximized at 11 am, remained constant until 3 pm and then decreased. This was probably due to water stress. Once the cross-sectional areas of the xylem active in transporting water has been determined at the Davis and Oakville sites, values of grapevine transpiration will be determined and basal crop coefficients estimated.