黄土高原陆面水分的凝结现象及收支特征试验研究

An experimental study of land surface condense phenomenon and water budget characteristics over the Loess Plateau

中国黄土高原是全球独特的地理区域,其陆面水分过程比较特殊。利用黄土高原陆面过程试验研究(LOPEX)的陇中黄土高原定西陆面过程综合观测站的资料,分析了陆面水分凝结现象及其出现频率与局地微气象条件的关系,研究了露水(霜)量及其出现频率的季节分布特征以及受降水和天气阴、晴的影响规律。同时,对比分析了降水、露水、雾水和土壤吸附水对陆面水分的贡献率,讨论了涡动相关法、蒸渗计和蒸发皿观测的陆面蒸发量的差别及其与陆面水分来源的年平衡关系,给出了半干旱区陆面水分平衡的日循环特征。发现露水对风速、大气湿度、近地层温度梯度的依赖很强,一般在风速为1.5 m/s、相对湿度大于80%和逆温强度为0.25℃的情况下露水(霜)量最大;刚降水后的晴天露水量比较大;实际蒸散量与蒸发力的差距十分明显,陆面水分平衡特征表现为一个"呼吸"过程。

The Chinese Loess Plateau is a unique area in the world,dominated by a semi-arid climate with unusual land surface hydrology.The previous studies have paid little attention to the non-precipitation land surface hydrological processes,such as dew formation,on the semi-arid Loess Plateau.Therefore,it is important to improve our understanding of the micrometeorological conditions controlling dew formation and the main characteristics of the land surface water balance.Here,we have analysed the observation data on land surface processes,collected at the Dingxi observational station in the LOPEX experimental area,to investigate the relationships between dewfall（or frost） amount and micrometeorological conditions,the characteristics of the seasonal distribution of dewfall（frost） and the influence of precipitation and other weather conditions on dewfall（frost）.The fractional contributions of precipitation,dewfall,fog and soil-adsorbed water to the land surface water balance are also calculated.The differences between land surface evapotranspiration rates observed by the three methods（eddy-covariance,large lysimeter and evaporation pan）,and the balance between evapotranspiration and the total land surface water sources are discussed.The land surface water sources,without precipitation or evapotranspiration,jointly constitute a diurnal cycle known as ＂water respiration＂ on the land surface.It has been found that the amount of dewfall（frost） is influenced by micrometeorological conditions： a higher relative humidity of the atmosphere surface layer leads to greater dewfall（frost）.The amount of dew（frost） is greatest under an inversion strength of 0.25℃ and wind speed of 1.5 m/s,explaining why dewfall totals are the greatest in autumn,second-greatest in spring and the lowest in summer and winter.Over an annual cycle,the total land surface water sources are similar to evapotranspiration values observed by both the methods of lysimeter and eddy covariance,indicating that the land surface water budget is close to a steady state.In the semi-arid region of the Loess Plateau,precipitation is still the dominant water source,but non-precipitation surface water can account for more than about 15% of the total land surface water sources.Of these non-precipitation land surface water sources,soil absorbed water is the greatest,followed by dew,and fog is the weakest.