采用气候倾向率方法,对西藏雅鲁藏布江(下称"雅江")中游1981~2005年14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25年西藏雅江中游年蒸发皿蒸发量在流域绝大部分站点均呈现显著的减少趋势,平均减幅为109.92mm,以夏季减少趋势最明显。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(0.52℃/10a)明显比平均最高气温的升温速率(0.23℃/10a)大,导致气温日较差减少(-0.29℃/10a)。因此,雅江中游年日照时数和平均风速的显著下降,以及年平均相对湿度的明显增加可能是年蒸发皿蒸发量显著下降的主要原因,平均气温日较差的显著减小和降水量的增加在蒸发量减少趋势中的作用也不可忽视。
Using the data of monthly 20cm pan evaporation and its impact climatic factors(for instance, sunshine duration, wind speed, relative humidity, precipitation and daily temperature, et al.)at 14 meteorological stations in middle reaches of the Yarlung Zangbu River over Tibet from 1981 to 2005,the variations of annual and seasonal pan evaporation and the trend were analyzed by using the method of linear trend analysis. Main results were as follows:(1)Significant negative trend of annual pan evaporation were detected at the most stations, and the mean rate of decrease was 109.92 mm /10a (99% confidence level),especially in summer (-51.79 mm /10a, 99% confidence level). In terms of spatial distribution, the trend of annual pan evaporation increased in Milin county where the rate of increase was 41.22 mm /10a (95% confidence level),but the decreasing trends were found at the other stations,the rate of decrease was (1.23~348.68) mm/10a, which is obvious in Gongga county. (2)Significant negative trends of annual sunshine duration and wind speed were found in middle reaches of the Yarlung Zangbu River over Tibet. Significant positive trends of annual mean relative humidity and precipitation were detected. Significant increasing trend of annual mean temperature was detected, an increase of 0.33℃/10a. While the increasing trend of annual mean minimum temperature(0.52℃/10a)was higher than that of annual mean maximum temperature(0.27℃/10a), and the decreasing trend of daily temperature was significant(-0.29℃/10a).(3)It is thus obvious that the decrease of annual pan evaporation was mainly caused by the significant decrease of the annual sunshine duration and wind speed as well as obvious increase of annual mean relative humidity, the daily temperature and precipitation may also play an important role in an unknown mechanism.
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