AbstractIn this study, we aimed to clarify spatial variations in xylem sap flow, and to determine the impacts of these variations on stand‐scale transpiration (E) estimates. We examined circumferential and radial variations in sap flow velocity (Fd) measured at several directions and depths in tree trunks of black locust (Robinia pseudoacacia) and native oak (Quercus liaotungensis), both of which have ring‐porous wood anatomy, in forest stands on the Loess Plateau, China. We evaluated the impacts of circumferential variations in Fd on stand‐scale transpiration estimates using a simple scaling exercise. We found significant circumferential variations in Fd in the outermost xylem in both species (coefficients of variation = 20–45%). For both species, Fd measured at the inner xylem was smaller than that of the outermost xylem and the Fd at the depth of > 10 mm was almost zero. The simple exercises showed that omitting circumferential variations in Fd affected the E estimate by 16–21%, which was less than the effects of omitting within‐tree radial and tree‐to‐tree variations in Fd in both species. These results suggest that circumferential variations in Fd can be a minor source of error for E estimates compared with within‐tree radial and tree‐to‐tree variations in Fd, regardless of the significant circumferential variations. Copyright © 2011 John Wiley & Sons, Ltd.