Evapotranspiration(ET) within an ecosystem is crucial for the water-limited environment that currently lacks adequate quantification in the arid region of Northwest China, mainly covered by phreatophytes, such as the Populus euphratica Oliv. tree and the Tamarix ramosissima Ledeb. shrub species. Accordingly, ET was measured for an entire year using eddy covariance(EC) in P. euphratica stands in the lower Heihe River Basin, Northwest China. During the growing season,the total ET was 850 mm, with a mean of 4.0 mm/d, which is obviously more than that observed at tree-level and standlevel scales, which was likely due to the different level of soil evaporation induced by irrigation via water conveyance.Factors associated with ET fall into either environmental or plant eco-physiological categories. Environmental factors account for at least 79% variation of ET, and the linear relationship between ET and the groundwater table(GWT) revealed the potential water use of P. euphratica forests under the non-water stress condition with the GWT less than 3 m deep.Plant eco-physiological parameters, specifically the leaf area index(LAI), have direct impact on the seasonal pattern of ET, which provides a valuable reference to the wide-area estimates of ET for riparian forests by using LAI. In conclusion,P. euphratica forests have high water use after water conveyance, which may be the result of long-term adapting to local climates and limited water availability. Evapotranspiration (ET) within an ecosystem is crucial for the water-limited environment that currently lacks adequate quantification in the arid region of Northwest China, mainly covered by phreatophytes, such as the Populus euphratica Oliv. Tree and the Tamarix ramosissima Ledeb. Shrub species. During the growing season, the total ET was 850 mm, with a mean of 4.0 mm / d , which is obviously more than that observed at tree-level and standlevel scales, which was likely due to the different level of soil evaporation induced by irrigation via water transport. Factors associated with ET fall into either environmental or plant eco-physiological categories. Environmental factors account for at least 79% variation of ET, and the linear relationship between ET and the groundwater table (GWT) revealed the potential water use of P. euphratica forests unde r the non-water stress condition with the GWT less than 3 m deep. Plant eco-physiological parameters, specifically the leaf area index (LAI), have direct impact on the seasonal pattern of ET, which provides a valuable reference to the wide- area estimates of ET for riparian forests by using LAI. In conclusion, P. euphratica forests have high water use after water conveyance, which may be the result of long-term adapting to local climates and limited water availability.