Fig. 6. Performance of a representative heat-balance sap flow gauge operated under variable power control using an open-loop algorithm. The gauge was on an 11-yr-old deficit-irrigated Cabernet Sauvignon vine in a semiarid environment. (A) Power to the gauge heater (Q); (B) temperature difference between upstream and downstream thermopiles (T); (C) sap flow and global irradiance (R_s). Data were recorded on DOY 237, 2003. Horizontal reference line in (B) is T_target (1.5°C).