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The oxygen isotope compositions of ecosystem water pools and fluxes are useful tracers in the water cycle. As part of the Heihe Watershed Allied Telemetry Experimental Research (HiWATER) program, high-frequency and near-continuous in situ measurements of O-18 composition of atmospheric vapor (delta(v)) and of evapotranspiration (delta(ET)) were made with the flux-gradient method using a cavity ring-down spectroscopy water vapor isotope analyzer. At the sub-daily scale, we found, in conjunction with intensive isotopic measurements of other ecosystem water pools, that the differences between O-18 composition of transpiration (delta(T)) and of xylem water (delta(x)) were negligible in early afternoon (13:00-15:00 Beijing time) when ET approached the daytime maximum, indicating isotopic steady state. At the daily scale, for the purpose of flux partitioning, delta(T) was approximated by delta(x) at early afternoon hours, and the O-18 composition of soil evaporation (delta(E)) was obtained from the Craig-Gordon model with a moisture-dependent soil resistance. The relative contribution of transpiration to evapotranspiration ranged from 0.71 to 0.96 with a mean of 0.87 +/- 0.052 for the growing season according to the isotopic labeling, which was good agreement with soil lysimeter measurements showing a mean transpiration fraction of 0.86 +/- 0.058. At the growing season scale, the predicted O-18 composition of runoff water was within the range of precipitation and irrigation water according to the isotopic mass conservation. The O-18 mass conservation requires that the decreased delta O-18 of ET should be balanced by enhanced delta O-18 of runoff water. (C) 2015 Elsevier B.V. All rights reserved.