Partitioning evapotranspiration flux components in a subalpine shrubland based on stable isotopic measurements

Abstract

Soil, vegetation and atmospheric water vapor (0.1∼3 m above ground) were sampled in a subalpine shrubland covered with Quercus aquifolioides during three days in the early monsoon period in Wolong Nature Reserve, China. In June 2006, the average LAI of Q. aquifolioides was 2.05 m2 m-2 and the average community coverage was more than 90%. Isotope turbulent mixing relationships, isotopic values of transpired water from plants and that of evaporating water vapor from soil surface were used to estimate fractions of transpiration and evaporation contributing to the total evapotranspiration (ET). The method worked well for δD, but it was imprecise for δ18O because the minute isotopic differences between transpired water and soil water. The results from δD showed that fractional contributions of plant transpiration to ET were 74.5±9.9%, 65.6±8.3% and 96.9±2.0% on 21st, 24th and 25th June, 2006, respectively, implying that ET is mostly generated by plant transpiration. Notably, the transpiration from herbage layer for ET was likewise important as that from shrub layer. Our approach is useful for partitioning ET in semiarid subalpine shrubland at an ecosystem scale on short time steps. This approach improves the understanding of water exchange in semiarid ecosystems, and offers an opportunity to measure and validate component fluxes with accurate spatial representation at a common scale.