New Discussion on Dry Valley Formation in the Three Parallel Rivers Region
三江并流区干热河谷成因新探析

Dry valleys are noticeable and unique geological-ecological phenomena in the Qinghai-Tibet Plateau periphery as well as in the \!Three Parallel Rivers\" region,which were formed by a series of synthetic impacts,including tectonics-geomorphology-palaeoecology effect,air circulation-monsoon-narrow pipe effect,topographical wave-precipitation-local circulation-fohn effect and vegetation-human activity-land use-disturbing effect.In the early Tertiary,the \!Three Parallel Rivers\" region was a component of the broad dry climate zone in China.With the uplifting of the Qinghai-Tibet Plateau and the evolution of the southwest monsoon,the climate and vegetation in the region gradually changed into the subtropical climate.Since the end of Pliocene,the longitudinal Range-Gorge landforms formed.Great rivers eroded back to the source,and valleys incised deeply.At the foot of the deep-incised valleys,the arid or semi-arid ecological landscapes that are equal to their latitude formed,which in the sense of palaeoecology has realized the reversion.In winter,influenced by the south branch of the Westerly the climate in the region is dry.In summer,the region is controlled by the southwest monsoon;in spite of much vapor the region still has formed four areas with less precipitation due to the special landforms structure.Part of airstreams flow northward quickly without any obstruction along the narrow and straight valley.With the increasing of altitude topographical precipitation formed.The wavy distribution curve of precipitation formed on the longitudinal-arranged underlying range-gorge landforms.At leeside of range,dry valley formed due to fohn effect.On the original background of fragile ecosystem,the social economy development strengthened the intensity of the land use,reduced the vegetation and aggravated the loss of water and soil erosion,which made the dry valley develop to further dry.These proved that the dry valley was the result of coupling effect among land,air,water,biome and human.