| Evapotranspiration,which includes evaporation and transpiration,is a core component of the ecosystem water cycle.The canopy composed of tall and dense branches and leaves of Picea crassifolia forest ecosystem in the upper reaches of Heihe River intercepts the meteoric precipitation and redistributes it.Therefore,understanding and analyzing the evapotranspiration process is of great significance to reveal the regional water cycle and eco-hydrological process.This study was carried at the Pailugou Picea crassifolia forest ecosystem of the upper reaches of the Heihe River Basin,Northwest China.Based on stable oxygen isotopic composition(?18O)of soil water,plant xylem water and atmospheric water vapor at both 2700 and 2900m altitude in June and September,2009?2700m-June,2900m-June and 2900m-September?.?18O of soil evaporation moisture(?18OE),plant transpiration moisture(?18OT)and evapotranspiration(?18OET)were modeled by theCraig-Gordon model,isotope steady state hypothesis and Keeling Plot model.In addition,we partitioned evapotranspiration of this forest ecosystem into evaporation and transpiration combined multiple linear mixed model.The results showed that:?1?Isotope fractionation occurs during soil water evaporation processes with obviously negative ?18OE in soil evaporation moisture.The ?18OE in 2700m-June,2900m-June and 2900m-September varied from-35.0 to-25.2‰,-35.9 to-30.4‰ and-33.2 to-29.0‰,respectively.The ?18OT in 2700m-June,2900m-June and 2900m-September ranged from-7.0 to-4.2‰,-9.0 to-6.3‰ and-8.8 to-6.6‰,respectively.The ?18OET in 2700m-June,2900m-June and 2900m-September were-18.5%‰,-12.9‰ and-10.2‰,respectively.The order was ?18OT>?18OE>?18OE,which was according with the assumption of isotope steady state.?2?The contribution rate of transpiration to evapotranspiration?fT?in the 2700m-June,2900-June and 2900m-September varied from 34.9 to 58.2%,74.1 to 84.7%and 66.8 to 93.9%,respectively.Contribution rate of evaporation to evapotranspiration?fE?in the 2700m-June,2900-June and 2900m-September varied from 41.8 to 65.1%,15.3 to 25.9%and 6.1 to 33.2%,respectively.Our results indicated that fT was higher than that of fE,suggesting that most evapotranspiration came from transpiration,and plant transpiration was a vital part of evapotranspiration in the Picea crassifolia forest ecosystem of the upper reaches of the Heihe River Basin?3?fT was negatively correlated with temperature and positively correlated with humidity with non-significant,indicating temperature restrained fT and humidity promoted fT.In addition,various environmental factors such as solar radiation,wind speed,air pressure,local turbulent diffusion,atmospheric entrapment,air mass advection,and biological factors such as the growth of vegetation themselves are likely to have a common impact on fT,and the specific impact mechanism remains to be further explored.The results of this paper can provide a theoretical basis for further study on the regional circulation and watershed scale water circulation of Heihe River Basin. |
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