藏北高原蒸发皿蒸发量及其影响因素的变化特征

Changes of pan evaporations and its impact factors over northern Tibet in 1971-2006

采用气候倾向率方法,对藏北高原1971-2006年6个气象站年、季小型蒸发皿蒸发量及其影响气候因子的变化趋势进行了分析。结果表明:近36年藏北高原年蒸发皿蒸发量各站点均呈现显著的减少趋势,平均减少61.7mm/10a(通过99%显著性检验),以夏季减幅最明显。就地域分布而言,蒸发皿蒸发量的下降主要表现在藏北高原的中西部。蒸发量减少幅度随经度的增加减小,随海拔高度的升高而加大。影响蒸发皿蒸发量的主要气候因子日照时数、平均风速呈现显著下降趋势,平均相对湿度、降水量表现为显著增加,平均气温显著升高,平均最低气温的升温速率(0.54℃/10a)明显比平均最高气温的升温速率(0.17℃/10a)大,导致气温日较差的减小(-0.37℃/10a)。藏北高原平均气温日较差和日照时数的显著减小,以及平均相对湿度的明显增加可能是蒸发皿蒸发量显著下降的主要原因,降水量的增加和平均风速的明显减小在蒸发量减少趋势中的作用也不可忽视。

With the data of monthly 20cm pan evaporation and its impact climatic factors at 6 meteorological stations over northern Tibet from 1971 to 2006, the trend is analyzed through the linear trend analysis. The main results are as follows ： （ 1 ） Significant negative trend of annual pan evaporation is detected at all stations of northern Tibet, and the mean rate of decrease is 61.7 mm per 10-year （99% confidence level）, especially in summer （ -20.8 mm per 10-year, 99% confidence level）. The spatial distribution of the change rate shows that the decrease of annual pan evaporation mainly occurs in mid-western of northern Tibet, the decreasing rate declines with the increase of longitude and rises with the elevation of altitude. （2） Significant decrease trends of annual sunshine duration and wind speed are found over northern Tibet. Significant positive trends of annual mean relative humidity and precipitation are detected. So is significant increase trend of annual mean temperature, with its increase is 0.34℃ per 10-year. While the increase trend of annual mean minimum temperature（0.54℃ per 10-year）is bigger than that of annual mean maximum temperature（0.17℃ per 10-year）, and the trend of daily temperature range decrease significantly（ -0.37℃ per 10-year）.And （3）it is thus obvious that the decrease of the pan evaporation is mainly caused by the significant decrease of the daily temperature range and sunshine duration as well as the obviously increase of the mean relative humidity. Precipitation and wind speed may also play an important role in the unknown mechanism.