三江并流区干热河谷成因新探析

干热河谷是青藏高原周边及三江并流区引人注目的、较为独特的地生态现象。干热河谷是构造-地貌-古生态效应、环流-季风-“狭管”效应、地形波-局地环流-降水-焚风效应、植被-土地利用-人类活动干扰效应等综合作用而形成的原生性的地生态现象。早第三纪时,三江并流区是斜贯我国广阔干旱气候带的组成部分。随着青藏高原的隆起和西南季风的形成演化,逐渐向暖湿性的亚热带气候和植被演化,进入上新世末-更新世以后,纵向岭谷地貌形成,大江大河溯源侵蚀,河谷深切,在深切河谷下部逐步形成与其纬度位置相当的干旱半干旱生态景观,从古生态上实现了某种意义上的“返祖”现象。冬半年本区受西风南支急流控制形成干季,夏半年受西南季风控制,尽管带来大量水汽,但受特殊地貌格局影响,在本区形成金沙江河谷等四个少雨区,部分气流沿狭窄平直河谷急速北流一路无阻,水汽被输送到北部地势增高之处形成地形降雨。纵向排列的岭谷地形波,形成明显的降水波状起伏分布曲线,气流越过山岭后下沉气流带来“焚风”效应,形成干热河谷。人类活动叠加在原已生态脆弱的干热河谷生态系统之上,社会经济发展使其土地利用强度加剧、植被减少、水土流失严重等,促使生态环境进一步朝干热化方向发展。因而,干热河谷形成有着其地史、大气环流、季风、大地貌及地形波效应、植被抗逆性差等自然要素的综合作用,并叠加了人类的干扰,是局地地-气-水-生-人交互作用及耦合效应的综合产物。

New Discussion on Dry Valley Formation in the Three Parallel Rivers Region

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.