Long-term dynamics of dry valleys in the upper reaches of Mingjiang River, China
岷江上游干旱河谷区域空间变化的定量判定

The dry valleys in Hengduan Mountains of Southwesten China are located in unique geographical regions where ecological security and environment conservation are crucial for local sustainable development.Depending on the microclimatic(thermal)conditions,vegetation characteristics and soils,dry valleys in the region can be divided into three major types:the dry-hot valley,the dry-warm valley and the dry-cool valley.The dry valleys in the upper reaches of Minjiang River are the dry-cool valley with increasing population and rapid economy growth,serious soil erosion,droughts,deterioration,and desertification have greatly shaped the ecosystem structure,function,and service of the dry valleys across the region.In this study,we quantified the distribution of the Dry Valleys in the Upper Reaches of Mingjiang River,using geographic information system(GIS)and various remote sensing products.A special attention was paid to the changes of the total area of dry valleys and the upper boundaries of the valleys.We found that the total area of dry valleys increased from 93140 hm2 in 1974 to 123078 hm2 in 2000;and the change rate from 1995 to 2000 was lower than that from 1974 to 1995.The upper boundaries of the valleys also ascended rapidly with the upper limit of 3128 m in 1974,3167 m in 1995(i.e.39 m increase),and 3181 m in 2000-an average of 2 m per year over the 26-year study period.We conclude that vegetation recovery and ecological constructions are urgently needed to slow the expansions of these dry valleys.