Fig. 1. Typical daytime course of (A) power (Q); (B) temperature difference between upstream and downstream thermopiles (T), with dashed line representing T_target of 1.3°C.; and (C) sap flow and global irradiance (R_s) under clear skies for a representative heat-balance sap flow gauge on a 10-yr-old well-watered Concord vine in a semiarid environment. The gauge was operated under variable power control using a PD algorithm. Data were collected DOY 202, 2000. Overnight, Q was held at 0.2 W to avoid overheating the stem.