Partitioning oak woodland evapotranspiration in the rocky mountainous area of North China was disturbed by foreign vapor, as estimated based on non-steady-state 18O isotopic composition.

Highlights: [•] Calculating and validating Q. variabilis transpiration δ T under NSS conditions. [•] δ ET of ecosystem obtained by Keeling curve showed an n-shaped variation in sunny day. [•] Transpiration also accounted for majority of ET under NSS conditions in the hilly area. [•] Vapor concentration was the key factor in partitioning evapotranspiration with isotope method. [•] Foreign vapor caused more uncertainties in the isotopic ET partitioning. [ABSTRACT FROM AUTHOR]