Isotopic Composition And Simulation Of Evapotranspiration Components In A Temperate Deciduous Forest

As a main water and energy transfer process in the SPAC (soil-plant-atmosphere continuum) system, evapotranspiration plays an important role in global water cycle,CO2 hydration and ecosystem productivity. Ecosystem hydrogen and oxygen stable isotopes can provide unique traceable information for the studies about the water exchange between land and atmosphere and the hydrological cycle in terrestrial ecosystem. SiLSM is coupled by a land surface model with the isotopic enrichment model, and it could investigate the characteristics of water exchange between land and atmosphere, but ignore soil evaporation. The addition of soil evaporation isotopic model could help us understand the characteristics of evapotranspiration components and their isotopes, and explore the effects of evapotranspiration on atmospheric isotopes. In this study, a land surface process model (S-W model) was combined with leaf water isotope enrichment model and soil evaporation model, called the second SiLSM model (SiLSM2), to partition evapotranspiration and simulate isotopic composition of evaporation, transpiration and ET.Based on the experimental data of Canadian deciduous forest ecosystem in the growing season of 2009, the temporal variability and spatial pattern in isotopic composition for various ecosystem water pools and water vapor were analyzed; the variability of the isotopic composition in evapotranspiration and soil evaporation as well as their isoflux were analyzed. Further, The main factors controlling vapor d-excess and the main processes influencing the distribution of vapor isotopes were discussed. The results indicated that the isotopic compositions varied in different water pools; ?18Ov and ?Dv decreased with increasing height, showing an W-shaped diurnal variation. Atmospheric humidity is the main factor that influence the isotopes distribution, as water vapor mixing ratio and relative humidity were all significantly correlated with dv negatively. While evapotranspiration rate showing a significant positive correlation with dv.The diurnal variation of leaf water isotopic composition simulated by SiLSM2 issmaller than that by SiLSM, but similar at noon. The simulated ?18OET of the two models differ little during daytime. SiLSM2 model performs well on simulated leaf water 18O-H218O on canopy scale at midday. Daytime ?18OET is lower than night,showing a U-pattern diurnal variation, simulated midday ?18OET are slightly underestimated compared with flux gradient value. Simulated evapo-transpired18O-H218O isoflux is lower than observed value while both of them are positive,indicating that evapotranspiration could promote the enrichment of isotopic composition of water vapor. The observed evaporated 18O-H218O isoflux contribute positively to vapor isotopes enrichment while the simulation shows an opposite result.Although much transpired flux contributing to enrich the air with 18O-H218O, the heavy isotopes of vapor are more depleted in daytime. Therefore, it could be concluded that evapotranspiration enriched effect on the water vapor 180 could not counteract the depletion brought by other processes such as atmospheric entrainment.Understanding the response of dv to terrestrial water flux contribution need to be further research that combined with D-HDO simulation by SiLSM2 model.