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The knowledge of plant water use characteristics was essential for ecosystem management and water resources distribution in arid inland river basin. This study was conducted to quantify the sap flow variations and water use sources of shelter‐belt trees at the oasis–desert ecotone in the middle of China's Heihe River Basin. Sap flow was measured by the thermal dissipation method on eight Gansu Poplar (Populus gansuensis) trees with different diameter at breast height in 2012 and 2013. The mean sap flow density increased linearly with diameter at breast height. The sap flow density exhibited linear relationship with solar radiation, and it increased logarithmically with vapour pressure deficit and air temperature, whereas the water table had negative impact on sap flow. The relationship between sap flow and soil relative extractable water at 0–220 cm depth was implicit during the whole growing season, whereas the soil water had apparently positive influence on sap flow after the shelter‐belt trees irrigation. The stand transpiration rate represented as a logarithmic function of reference crop evapotranspiration. The total transpiration of Gansu Poplar during growing season was 599.3 mm, whereas only 58.1% of which was provided by the precipitation and irrigation. The groundwater and cropland irrigation were critical water sources of shelter‐belt trees. The contribution of groundwater to tree transpiration was estimated to be 35.1 and 19.0% in 2012 and 2013, respectively. The great precipitation in 2013 weakened the dependence of tree transpiration on groundwater. The estimated threshold distance of cropland irrigation influencing the tree transpiration was about 8 m. Copyright © 2015 John Wiley & Sons, Ltd.