Abstract: Dewfall can be a substantial water resource for desert ecosystems,and it is crucial to maintain water balance. However,little is known about the formation,quantitative characteristics,and influencing factors of dew in extreme arid region. In order to answer these questions,this study constructed a positioning observation field in the lower reaches of Tarim River,Xinjiang,China. The data series,including latent heat flux,3D ultrasonic wind velocity,temperature,humidity,radiation and the soil surface moisture,were automatically collected by the eddy covariance(OPEC EC150) system in the typical Populus community during June 10 to December 31, 2013. Based on the observation data and energy balance,this paper analyzed the occurrence days,daily duration and quantity of dewfall. And what's more,this paper constructed the model framework which took the surface temperature and dew point temperature as the core,considering the nonlinear fitting relationship between dew duration, intensity and meteorological factors. The results show that the number of dewfall days was 142 days,accounting for 81% of the total observation days(182 days). The average daily dewfall duration was 3-4 hours, the daily dew value was 0.13 mm and the cumulative dew amount was 19.78 mm during observation period. The days and amount of pure canopy condensation were 47 days and 8.5 mm which were obviously higher than that of the pure soil surface condensation,meaning that the role of canopy condensation was more important than the soil surface condensation in growing season. This paper also constructed assessment methods to simulate the occurrence days,daily duration and the daily intensity of dewfall. The assessment results indicate the methods have good simulation accuracy at long-time scale,such as the month time scale. Because of the actual dewfall amounts was 0.48 mm higher than the precipitation during June to December,and the dewfall amount reached 75% of the total precipitation in 2013,the dewfall should play an important role in the regional water balance. The highlights of this paper mainly include three aspects:(1) the observation results of eddy covariance system have higher precision and reliability compared to the classical methods,such as the micro-lysimeters and artificial condensation surface methods;(2) the research perspective was not only confined to the ground(soil surface),it focused on the canopy condensation and revealed the canopy condensation was the main form of dewfall in plant grow season;(3) this research proposed an assessment model which can well simulate the frequency and quantity characteristics of dewfall at monthly time scale. The results of this study confirmed that dewfall was an imprint branch of regional water balance and have positive effect to plant communities. The observation method and model framework of this research will provide method reference and data support to further study.

Key words: dewfall, canopy condensation, eddy covariance, quantitative assessment, water balance