The accurate estimation of evapotranspiration (ET) in arid regions is important for improving the water use efficiency of vegetation. Based on successive observations from May to October of 2014, we estimated the ET of a Populus euphratica Oliv. forest during the growing season in an extremely arid region using the PM (Penman-Monteith), SW (Shuttleworth-Wallace) and SSW (an improved canopy transpiration model) models. Estimated ET values were compared with those of the eddy covariance measurements. Results indicated that the actual ET of the P. euphratica forest was always overestimated by the PM model. The accuracy of the SW model was higher than that of the PM model. However, some data were not easily obtained because of the complicated structure of the SW model. The newly proposed SSW model gave the most accurate ET values, and its accuracy was higher at hourly than at daily time scale. In conclusion, the SSW model is more suitable for sparse vegetation system at large scales in extremely arid regions.
Evapotranspiration (ET) within an ecosystem is crucial for die water-limited environment that currently lacks adequate quantification in the arid region of Northwest China, mainly covered by phreatophytes, such as the Populus euphratica Oliv. tree and the Tamarix ramosissima Ledeb. shrub species. Accordingly. ET was measured for an entire year using eddy covariance (EC) in P. euphratica stands in the lower Heihe River Basin, Northwest China. During the growing season, the total ET was 850 mm, with a mean of 4.0 mm/d, Which is obviously more than that observed at tree-level and stand level scales, which was likely due to the different level of soil evaporation induced by irrigation via water conveyance. Factors associated with ET fall into either environmental or plant eco-physiological categories. Environmental factors account for at least 79% variation of ET and the linear relationship between ET and the voundwater table (GWT) revealed the potential water use of P. euphratica forests under the non-water stress condition with die GWT less than 3 m deep. Plant eco-physiological parameters, specifically die leaf area 'index (LAI), have direct impact on the seasonal pattern of ET which provides a valuable reference to the wide-area estimates of ET for riparian forests by using LAI. In conclusion, P. euphratica forests have high water use after water conveyance, which may be the result of long-term adapting to local climates and limited water availability.
