Fig. 3. Performance of a representative heat-balance sap flow gauge operated in constant power mode on a 10-yr-old well-watered Concord vine in a semiarid environment. (A) Power to the gauge heater (Q); (B) temperature difference between upstream and downstream thermopiles (T), with dashed line representing T of 1.3°C; (C) sap flow and global irradiance (R_s). The substantial error that can occur due to small T is evident around 10.00 LST. Data were collected on DOY 272, 2000.