| Tibetan Plateau is known as the water tower of Asia due to its abundant water resources in this region.Glaciers and lakes are two important components of the surface system of the Tibetan Plateau.They are also important freshwater resources.Glaciers and lakes in the Tibetan Plateau are sensitive and exhibit an immediate response to climate forcing.The changes of glaciers and lakes are closely related to water resource changes and geological disasters.Under the background of global warming,glaciers have shown a trend of accelerated retreat in the Tibetan Plateau and its surrounding areas.Specifically,southeast Tibetan Plateau is one of the regions with the largest range of glacier change.The marine glaciers in this region are important indicators of climate change.Glacial retreat may lead to corresponding changes of lakes in the down stream area,which may increase the risk of glacial lake outburst.The glacier retreat and lake change have strong correlation with the climate variation.Therefore,it has great significance to carry out long-term dynamic monitoring of glaciers and lakes and to understand the climate implications of glaciers and lakes in the southeastern Tibetan Plateau.The rapid development of satellite remote sensing technology provides effective ways for large-scale and long-term monitoring of glaciers and lakes.The southeast region of the Tibetan Plateau was chosen as the study area.The glaciers and lakes in this region were mapped by using remote sensing image data in three periods(1995,2005 and 2015).The characteristics of the distribution and changes of glaciers and lakes during the study period were analyzed.By combining with the meteorological station data in 1995-2014,the possible reasons of the changes in glaciers and lakes in the study area were investigated.The relationship of the changes in glaciers and lakes were also discussed.The main results of this research are as follows:(1)In this study,we proposed an automatic classification method for glaciers and lakes in the southeast Tibetan Plateau based on multiple features of land surface,multi-temporal and multi-source remote sensing data.Debris-free and debris-covered glaciers were automatically classified by using a machine-learning algorithm along with the analysis of spectral,thermal,topographic and textural features based on multi-temporal Landsat images and multi-source digital elevation model data.The algorithm was implemented using multi-temporal images to minimize the influence of cloud cover and seasonal snow.Moreover,non-frozen lakes and frozen lakes were automatically mapped by using the improved global-local step-by-step iterative method.The improved algorithm was based on water index and automatic selection of the segmentation threshold of lakes.The effect of cloud,mountain shadow and glaciers could be minimized by using multi-temporal images combined with the glacier results of the same period and digital elevation model data.(2)The characteristics of glacier distribution and glacier variation in the study area from 1995 to 2015 were analyzed.Based on the extracted glacier results of three target years,the characteristics of glacier distribution and glacier variation in the study area were analyzed from four aspects,i.e.glacier size,elevation,aspect and debris coverage.The results demonstrated that the glaciers in the study area were melting,with a total area reduction of 2842.08 km~2 in the period from 1995 to 2015.The area change percentage was 29.9%of the glacier area in 1995 and the shrinking rate had accelerated in the past decade.The glaciers with large size class are characterized by larger area and less number than the glaciers with small size class in the study area.The glacier size greatly influenced the relative and absolute change of glacier area.The distribution of glacier area in this region presented a normal distribution with the increase of altitude.The glaciers were mainly distributed at altitudes from 4400 m to6000 m.The glaciers at the altitude of 5200 m-5400 m has the largest area.The glacier area at all altitudes had reduced during the past 20 years and the highest absolute loss of glacier area shifted to higher elevations.The retreat rate of glaciers with elevation above 4800 m accelerated in the past decade.A majority of glaciers were facing north.The glaciers in all aspects presented a decreasing trend in area and the glaciers facing southeast changed the most.Debris-covered glaciers generally located at lower elevation compared to debris-free glaciers.The debris-covered glaciers at the altitude of 4600 m-4800 m had the largest area.A general decreasing trend in area was noted for both debris-free glaciers and debris-covered glaciers from1995 to 2015.The sparsely supraglacial-debris-covered glaciers illustrated the highest recession rate and the recession rate accelerated in the recent decade.(3)The characteristics of lake distribution and lake variation in the study area from 1995 to 2015 were analyzed.Based on the extracted lake results of three target years,the characteristics of lake distribution and lake variation in the study area were analyzed from four aspects,i.e.lake size,elevation,aspect and lake type.The results showed that lakes in the study area expanded with an overall increase area of 27.13km~2.The area change percentage was 6.0%of the lake area in 1995.The increasing lakes in the study area were mainly the lakes with the area between 0.05-0.1 km~2,while the decreasing lakes were mainly the lakes with the area between 0.4-0.5 km~2.In the past 20 years,the area of the co-existing lakes in the study region presented an overall growth trend.The most significant expansion area of the expanded lakes and the newly formed lakes occurred at similar altitudes(4200 m-4600 m).However,the most significant decreasing area of the shrinking lakes and the disappearing lakes occurred at different altitudes.The expanding lakes in the study area was mainly facing north,while the decreasing lakes was mainly facing south.Both the glacial and non-glacial lakes in the study area increased in the past 20 years.The glacial lake had a higher growth rate than non-glacial lakes.In the past 10 years,the growth rate of glacial lakes increased while that of non-glacial lakes decreased.In the study area,both the glacier-connected and glacier-disconnected lakes were growing.The growth rate of glacier-connected lakes was higher than that of glacier-disconnected lakes.The growth rate of both two types of lakes accelerated in the recent decade.(4)By combining with the meteorological data of this study area,the characteristics of climate change were analyzed.The reasons of glaciers and lakes changes were investigated.The results showed that the annual mean air temperature of the study area showed a significant upward trend from 1995 to 2015,while the annual precipitation showed a fluctuating downward trend.The annual evapotranspiration in the study area presented a fluctuating upward trend in the last 15years(2000-2015).Combined with the analysis of meteorological data,it was noted that the continuous increase of air temperature and the decrease of precipitation may be the main climatic factors which may cause glacier melting.There was a good correlation between the increasing number of lakes and the retreat area of glaciers.The rising temperature and the increasing melt water of glaciers were the main reasons for the expansion of the glacial lakes. |
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