Fig. 5. Performance of an exemplary heat-balance sap flow gauge on deficit-irrigated Cabernet Sauvignon grapevines during 5 d encompassing the end of a weekly dry-down and rewatering. Irrigation was applied by drip (0.75 L h^–1 vine^–1) for 17 h overnight DOY 233 to 234 and 16 h overnight DOY 234 to 235. (A) Power to the gauge heater (Q); (B) temperature difference between upstream and downstream thermopiles (T); (C) sap flow and global irradiance (R_s). The gauge was operated under variable power control using an open-loop algorithm. Maximum allowable Q deliberately adjusted on DOY 233. Arrow in (A) denotes time at which program implemented the high-temperature "fail-safe" mechanism, reducing Q to 0.25Q for the remainder of DOY 234. Horizontal reference line in (B) is T_target. Data were collected during 2003.