塔里木河下游典型胡杨群落大气凝结水的形成规律与数量评估

Dew formation and assessment of Populus euphratica community in the lower reaches of Tarim River Basin

在极端干旱的塔克拉玛干沙漠边缘地区,凝结水可能是一种重要的水源。然而,目前对该地区凝结水的形成、数量等特征所知甚少。借助于涡动协方差观测系统于2013年6-12月对沙漠边缘胡杨林群落的水汽通量、气象要素等进行观测,目的是揭示凝结水的形成特征及其在区域水量平衡中的作用,初步探讨凝结水定量评估方法。研究结果表明：胡杨林群落具有明显水汽凝结现象,在整个182 d的观测期内,凝结水出现的天数达到了148 d,占整个观测期总日数的81%,累积凝结水量为19.78 mm;凝结水出现时,夜间平均持续时间为3-4 h左右,平均日凝结水量为0.13 mm。单一冠层凝结出现天数为47 d,凝结总量8.5 mm,明显大于单一的地面凝结（44 d,6.65 mm）,生长季冠层凝结是主要的。基于观测数据,构建了凝结水出现天数、日持续时间与凝结速率的评估方程,模拟结果在长时间尺度上（月、季）可较好的反映实测结果（R2〉0.9）。观测结果显示,凝结水总量超过同期降水量0.48 mm,并达到当年年降水量的84%,凝结水对水分收支的作用与年降水相当,在区域水量平衡中扮演了重要的角色。

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.