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Estimation of the transpiration rate for a tree is generally based on sap flow measurements within the hydro-active stem xylem. In this study, radial variation of sap flow velocity (J(s) ) was investigated at five depths of the xylem (1, 2, 3, 5 and 8 cm under the cambium) in three mature Xinjiang poplar (Populus alba L. var. pyramidalis) trees grown at the Gansu Minqin National Studies Station for Desert Steppe Ecosystem from May to October 2011. Thermal dissipation probes of various lengths manufactured according to the Granier’s design were installed into each tree for simultaneous observation of the radial patterns of J (s) through the xylem. The radial patterns were found to fit the four-parameter GaussAmp equation. The peak J(s) was about 27.02 +/- 0.95 kg/(dm(2).d) at approximately 3 to 5 cm deep from the cambium of the three trees, and the lowest J(s) appeared at 1 cm deep in most of the time. Approximately 50% of the total sap flow in Xinjiang poplar occurred within one-third of the xylem from its outer radius, whereas 90% of the total sap flow occurred within two-fifth of the xylem. In addition, the innermost point of the xylem (at 8-cm depth), which appeared as the penultimate sap flow in most cases during the study period, was hydro-active with J(s,8) of 7.55 +/- 3.83 kg/(dm(2).d). The radial pattern of J(s) was found to be steeper in midday than in other time of the day, and steeper diurnal fluctuations were recorded in June, July and August (the mid-growing season). Maximum differences between the lowest J(s) (J(s,1) or J(s,8) ) and the highest J(s) (J (s,3) or J(s,5) ) from May through October were 12.41, 17.35, 16.30, 18.52, 12.60 and 16.04 g/(cm(2).h), respectively. The time-dependent changes of J(s) along the radial profile (except at 1-cm depth) were strongly related to the reference evapotranspiration (ET0). Due to significant radial variability of J(s) , the mean daily sap flow at the whole-tree level could be over-estimated by up to 29.69% when only a single probe at depth of 2 cm was used. However, the accuracy of the estimation of sap flow in Xinjiang poplar could be significantly improved using a correction coefficient of 0.885.