Abstract:

Evapotranspiration is large enough to be a very important component of water budget in the desert-oasis ecotone in arid regions. A better understanding of the evapotranspiration processes and its driving mechanisms is of insightful significance for protecting the stability of vegetation in the ecotone. Here within a physically based framework describing vapor transfer between the underlying surface and atmosphere， the controls of energy and vapor transfer on evapotranspiration over the underlying surface are determined quantitatively from observations of eddy covariance， micrometeorology， and vegetation dynamics. Rainfall events and vegetation dynamics relieved significantly water stress of the underlying surface， even so， the underlying surface was highly coupled with atmosphere given small decoupling coefficient （e.g.， 0.14）. The equilibrium and imposed components contributed 65% and 35% of the gross evapotranspiration， respectively. Surface conductance exerted an overwhelming control of evapotranspiration rate， which can explain nearly 70% of the variation in daily evapotranspiration. The annual accumulated evapotranspiration （i.e.， 218 mm） exceeded the corresponding precipitation （i.e.， 89 mm） by around 129 mm， suggesting that groundwater plays a pivotal role in sustaining vegetation growth in the ecotone， and keeping groundwater level fluctuating within a narrow range of values is consequently a high priority for conserving vegetation in this ecotone.

Key words: evapotranspiration, surface conductance, decoupling coefficient, water budget, groundwater