| Glaciers are a critical component of the earth system and the present accelerated melting and retreat of glaciers has severe impacts on the environment and human well-being, including vegetation patterns, economic livelihood, natural disasters, and water-energy supplies. Changes of glaciers in mountainous regions are widely recognized as one of the best natural indicators of global climate change, and the decline in glacier extent in mountains and other regions contributes to sea level rise. The glaciers in the west of China are not only an important supply of the numerous rivers, but also a large solid freshwater resource, which affect the life of peoples of the western dry region. With the contex of global warming, most of glaciers in China are retreating and the glacier retreat is more aggravately after1980s. The glacier retreat will affect the runoff of rivers and also brings a tremendous impact on the sustainable development of arid regions in western China. Hunce, the new glacier inventory of China is a necessary work.In order to monitor the changes of the glaciers in the Gongga Mountain region on the south-eastern margin of the Qinghai-Tibetan Plateau, monsoonal temperate glaciers were investigated by comparing the Chinese Glacier Inventory (CGI), recorded in the1960s, with Landsat MSS in1974, Landsat TM in1989,1994,2005, and ASTER data in2009. The remote sensing data have been applied to map the glacier outline by threshold ratio images (TM4/TM5). Moreover, the glacier outlines were verified by GPS survey on four large glaciers (Hailuogou, Mozigou, Yanzigou, and Dagongba) in2009. The results show that the area dominated by the76glaciers has shrunk by11.3%(29.2km2) from1966to2009. Glacier area on the eastern and western slopes of the Gongga Mountains decreased by15.2km2(9.8%since1966) and15km2(14.6%since1966), respectively. The loss in glacier area and length is respectively0.8km2and1146m (26.7m/yr) for the Hailuogou glacier,2.1km2and502m (11.7m/yr) for the Mozigou Glacier,0.8km2and725m (16.9m/yr) for the Yanzigou Glacier, and2.4Wand1002m (23.3m/yr) for the Dagongba Glacier.Glacier size strongly affects the percentage loss in glacier area. From1966to2009, the area loss in the size classes <0.5km2,0.5～1km2,1～5km2,5～10km2, and>10km2, equals6.3%,10.8%,34.8%,21.3%and26.8%, respectively. The shrinkage of the glaciers in the size class of1～5km2contributes to about1/3of the total area loss. In comparing glaciers changes with altitude, the glaciers area loss in the altitude5100～5300m and>5700m is larger than the glaciers in others altitude. The mean slope of all glaciers in this region ranges from15°to45°, and glaciers with the mean slope of15～20°and35～40°(covering an area of37.9%in1966) exhibit the largest shrinkage. The area loss of glaciers with the mean slope of25～30°is the smallest than others slopes. The shrinkage of these glaciers in northwest and east orientation is stronger than others aspects in this region and the glaciers in southeast orientation keep a smallest shrinkage.When the glaciers are grouped according to size classes (according to their CGI area), it shows that glaciers in area of1～5km2had a more notable reduction in area than other sizes of glaciers. Although two small glaciers have vanished, the glaciers retreat with area<0.5km2is the slowest. The small glacier is more sensitive to the climate changes but the glacier with area <0.5km2is poorly coupled to the regional climate because of the local topographic setting. Furthermore, although the change of small glacier can be protected by local topographic setting, two smallest glaciers have vanished in the Gongga Mountains. This may be explained that the glacier is too small to a consistent accumulation zones.The rate of glacier area loss with median altitude>5700m is the largest, and the second is the median altitude5100-5300m. The glacier retreat become weaker with the increased slope until30°, and the rate of glacier retreat was similar when the slope exceeds30°. The glaciers with east, northwest and south orientation have an obvious shrinkage. Although we can not find systematically changes of glacier retreating correlating to topographic factors, the glacier size, slope and aspect would probably influence the rate of glacier retreating in the Gongga Mountains.The DEM data was generated from dual-frequency differential GPS (GPS) data surveyed in2009combined with DEM from aerial photographs acquired in1966and1989. The results show that the mean elevation changes from1966to2009of ablation area are-1.23±0.55,-1.02±0.55and-0.93±0.55m/yr in Hailuogou, Yanzigou (east slope) and Dagongba Glacier (west slope), respectively. In consequence, all the three glaciers in ablation areas show decreasing trends with volume reduction of0.076,0.073and0.054km3, for Hailuogou, Yanzigou and Dagongba Glacier, respectively. The shrankage of glacier elevation and volume in the period of1966-1989is weaker than that in1989-2009. The glacier ablation of YZG glacier is about3.25cm/day in the summer and the average ablation is about424cm/yr. The maximum ablation of YZG Glacier is at an altitude of3800-3900m not at the glacier terminus. Compared with other glaciers, the ablation of YZG Glacier is stronger than DGB Glacier (271cm/yr) but weaker than HLG Glacier (582cm/yr). Ice velocities measured by DGPS at32fixed stakes implanted in the ablation area increase with distance from the glacier terminus, ranging from38m/yr approaching the glacier terminus to a maximum of49m/yr at the head of ablation area.Decades of climate records obtained from three meteorological stations in the Gongga Mountains were analyzed to evaluate the impact of the temperature and precipitation on glacier retreat. During1966-2009, the mean annual temperature over the eastern and western slope of the Gongga Mountains has been increasing by0.34℃/decade and0.24℃/decade (1988～2009), respectively. Moreover, mean annual precipitation was only increasing1%in past50years. This evidence indicates that the warming of the climate is probably responsible for the glacier retreat in the study region. Compared the mean temperature with mean retreat rate of glaciers, the retreat rate is not related in a clear way to the mean temperature, while the mean precipitation shows a well corresponding relation with the retreat rate of glaciers. When the mean precipitation is high, the retreat rate of glacier is slow, and vice versa. The glacier retreat in the Gongga Mountains is similar to the same glacier type but faster than continental glaciers type in the west of China. However, we have procured many significative and interesting results. Many open questions still need to be solved (e.g. spatial resolution of remote sensing images; the different of fieldwork; accuracies of glacier mapping). In the future, the monitoring of the glacier changes will be a long-time and hard work, especially for alpine glaciers. |
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