Exploration of Reason of Runoff Decrease in the Source Regions of the Yellow River
黄河源区径流减少的原因探讨

The characteristic of climatic variation in the source regions of the Yellow River during 1960-2000 is analyzed,the evapotranspiration of the source regions is estimated,and the variations of vegetation and frozen soil are studied,then the reason of runoff obvious decrease after the 1990s is explored.The results indicate that air temperature in the source regions of the Yellow River has increased evidently since the mid 1980s,and the mean precipitation after 1990 in this area is lower than before.Climatic variation has a warming and drying trend.The estimation of evapotranspiration shows that evapotranspiration in the source regions varies inapparently,so the direct reason of runoff decrease is the reduction of precipitation.The intensity of precipitation weakens after the 1990s.This may be another important reason of runoff decrease.The variation of normal different vegetation index(NDVI) in the source regions shows vegetation has a deteriorating trend in the late 1990s.Frozen soil is degenerating seriously since the 1980s and the upper limit of frozen soil is descending gradually,while the lower limit of frozen soil is ascending.The degeneration of vegetation and frozen soil may cause water level of frozen crust descent,and water in near surface soil leakage into lower soil,which may also influence runoff to decrease.