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A study was conducted to determine the relationship between midday measurements of vine water status and daily water use of grapevines measured with a weighing lysimeter. Water applications to the vines were terminated on August 24th for 9 days and again on September 14th for 22 days. Daily water use of the vines in the lysimeter (ET LYS ) was approximately 40 L vine -1 (5.3 mm) prior to turning the pump off, and it decreased to 22.3 L vine -1 by September 2nd. Pre-dawn leaf water potential (W PD ) and midday W l on August 24th were -0.075 and -0.76 MPa, respectively, with midday W l decreasing to -1.28 MPa on September 2nd. Leaf g s decreased from *500 to *200 mmol m -2 s -1 during the two dry-down periods. Midday measurements of g s and W l were significantly correlated with one another (r = 0.96) and both with ET LYS / ET o (r = *0.9). The decreases in W l , g s , and ET LYS /ET o in this study were also a linear function of the decrease in volumetric soil water content. The results indicate that even modest water stress can greatly reduce grapevine water use and that short-term measures of vine water status taken at midday are a reflection of daily grapevine water use.

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