青海省三江源地区退化草地蒸散特征

了解三江源地区退化草地的蒸散状况有助于认识该地区退化生态系统水循环，对该地区的生态安全有着重要的意义。本研究利用涡度相关法观测青海省玛沁县小嵩草退化草地（３４．３５°Ｎ；１００．５０°Ｅ，海拔３９６３ｍ）的水热通量，对该地区２００６－２００８年全年的蒸散情况进行了特征分析。研究表明，２００６－２００８年三江源退化草地的蒸散值分别为４５２．２４，４７４．２４，４５９．５７ｍｍ，降水量分别为４６０．７，４９６．１，４８０．１ｍｍ，其中有７５％以上的降水分布在生长季，全年水平上蒸散量与降水量之比（ＥＴ／ＰＰＴ）为９５％ 以上，蒸散量与降水量基本持平。生长季日蒸散量１．８～１．９ｍｍ／ｄ，而非生长季日蒸散量＜０．６ ｍｍ／ｄ。蒸散日变化和年变化均为单峰型，每年６－７ 月蒸散量最大，为２．０～２．２ｍｍ／ｄ；日蒸散峰值出现在正午左右，其最大值生长季为０．２１ｍｍ／ｈ，非生长季为０．１０ｍｍ／ｈ。退化草地蒸散的主要环境因子是净辐射，其次为饱和水汽压亏和空气温度。随着生长季的推进，潜热对这些因素的敏感性逐渐增大。与未退化的矮嵩草草甸相比，退化草地生长季蒸散量较小，而非生长季蒸散量较大，这一结果表明，高寒草地的植被盖度驱动草地生态系统的蒸散。

Characterizing the evapotranspiration of a degraded grassland in the Sanjiangyuan Region of Qinghai province

The evapotranspiration(ET) of degraded grassland in the Sanjiangyuan region was investigated to further elucidate the water cycle in this region.The eddy covariance technique was used to measure evapotranspiration and sensible heat flux continuously for 3 years in this degraded grassland in Qinghai province,China.The annual precipitations(PPT) were 460.7,496.1,and 480.1 mm for the three consecutive years.The annual ETs were 452.24,474.24,and 459.57 mm,varying in accord with the annual PPT.About 75% of the annual ET occurred during the growing season from DOY121 to 300.The ratio of ET to PPT was greater than 95% during the study period,indicating that there is some balance between ET and PPT and the average in the growing season for each of the three years were 1.86,1.92,1.81 mm/d,respectively.However,the averaged ET was lower than 0.6 mm/d during the non-growing season.The annual and daily dynamics were single peak type curves.The maximum monthly ET rate was 2.0-2.2 mm/d,which occurred mainly in June or July.The daily peak appeared at noon and was 0.21 and 0.10 mm/h during the growing and non-growing seasons respectively.ET was most sensitive to net radiation during the growing season while the effect of air temperature and vapour pressure deficit were secondary.Compared with the non-degraded area of Kobresia humilis meadow,the ET of the degraded grassland was lower in the growing season but higher in the non-growing season.The lower vegetation cover was attributed to this result.