Abstract: The ThreeRiver Source Region (TRSR), the source region of Yangtze River, Yellow River and Mekong River, is located in the hinterland of the Qinghai-Tibetan Plateau. With a typical continental plateau climate and alpine vegetation ecosystem, the water and heat matching is a key to vegetation variation over region. In this study, monthly humidity indexes in TRSR were computed using groundobserved monthly air temperature, precipitation, wind speed, relative humidity and air pressure from 1971-2010, based on the method of Ecological and Meteorological Monitoring Standard which recommended by China Meteorological Bureau. The spatiotemporal variation of aridity/humidity conditions and its dominated factor were analyzed using the method of Empirical Orthogonal Function (EOF), linear correlation and partial correlation analysis. The results show a clear of opposite mode in the zone of northern toward southern and western toward eastern in TRSR. It was found that there is a wet tendency after 1990s and 21st century in the zone of northern and eastern respectively, but a continual dry tendency since 1980s, especially in the zone of southern and western with a remarkable decreased trend(-8%/10a), occurred in other area of TRSR. Aridity/humidity conditions are controlled by precipitation and relative humidity in generally, but the situation is changed evidently after Mid1990s in the most area of TRSR due to a significant warming, and temperature plays a key of factor to dominate a decreased trend of humidity index even if precipitation increases markedly since then. Under a scenarios of climate warming, it shows a tendency of heat and wet obviously in the zone of northern and eastern and heat and aridity in a certain extent in most area of TRSR.

Key words: Threeriver Source region, humidity index, climate change