黄河源区径流减少的原因探讨

Exploration of Reason of Runoff Decrease in the Source Regions of the Yellow River

分析了黄河源区1960-2000年气候变化特点，对蒸发进行了估算，并分析了植被和冻土的变化，对径流在20世纪90年代后明显减少的原因进行了探讨。结果表明，黄河源区气温在20世纪80年代中期后明显增加，降水在90年代偏少，气候向暖干方向发展，但蒸发变化不大，径流减少的直接原因是降水的减少；在90年代后降水强度的减弱也可能是径流减少的重要原因；归一化植被指数（NDVI）数据显示植被在90年代后期呈现退化的趋势，冻土在80年代以后表现出的明显的退化趋势，植被冻土的退化可以使得冻结层上水位下移，土壤水向土壤下层的渗漏增加，也会造成径流的减少。

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.