Trend of Surface Humid Index in the Middle Reaches of the Yarlung Zangbo River over Tibet in Recent 40 Years
雅鲁藏布江中游地表湿润状况的趋势分析

Humidity index can be expressed by two factors which indicate input and output of atmospheric moist under natural conditions:Precipitation and potential evaporation,represent the change of water and energy balance.In order to clarify the present situation of the environment and regional variations,research has been done on surface wetness in Tibet.The result provides scientific basis for exploring reasons of environmental change over the past 30 years in Tibet and further research on future climatic change.Based on data of monthly maximum temperature,minimum temperature,precipitation,wind speed,relative humidity and sunshine duration from four meteorological stations in the middle reaches of the Yarlung Zangbo River(Brahmaputra)over Tibet from 1961 to 2000,data of the maximum potential evaporation was computed by Penman,Monteith Model,and the surface humid index was obtained.The linear trend,inter,decadal variation and the seasonal difference of the surface humid index were analyzed,and the its relationship with precipitation and temperature was discussed.The results indicated that an obvious linear trend of the warming of annual and seasonal mean temperature existed during the last 40 years,especially the warming was more obvious in winter.Annual precipitation presented a decreasing trend in the first 20 years(1961-1980) and an obvious increasing trend in the last 20 years(1981-2000).Since 1981,precipitation has increased significantly in the flood season(from May to September),with a mean increase of 8.6mm/a.For potential evaporation in the 1960s and the 1970s,the increasing trend was more significant,but the most significant decrease occurred in the 1980s and the 1990s.Also,the rate of pan evaporation over the Yarlung Zangbo River had,on average,steadily decreased,it is particularly so from 1983 to 2000,with a decreasing rate of 22.1mm/a.From the 1960s to the early 1980s,the ground surface showed a drying trend.While from the mid to the late 1980s and the 1990s,the ground surface presented a wetting trend due to precipitation increase and the maximum potential evaporation decrease.The results showed that the cold wet occurred in the 1960s and the warm wett in the 1990s.