2. Qilian Shan Station of Glaciology and Ecologic Environment, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou, Gansu 730000, China
The Shiyang River Basin located at the eastern Hexi corridor, western China, which include eight tributaries originating from East Qilian Mountains, from west to east, there are Xida, Dongda, Xiying, Jinta, Zamu, Huangyang, Gulang, and Dajing rivers. Precipitation, glacier and snow melt are the main water supply for Shiyang River. About 3.6% water supply comes from glacier melt, and groundwater of plain terrain comes from mountain runoff transformation (Zhang, 2006; Kang et al., 2009 ). For nearly half a century, with the development of industry and agriculture, in the middle and lower reaches of Shiyang River, the demand of water resources has increased. Because of over-extraction of groundwater and utilization, the contradiction of water demand between upstream and downstream has become very acute. Excessive exploitation of groundwater has led to serious ecology and environment problems (Ma et al., 2004 ; Shen et al., 2005 ).
In recent years, many scholars have studied climate, agricultural ecology, water resource and runoff change within the Shiyang River Basin (Lan and Kang, 2000; Lan et al., 2003 ; Kang et al., 2004 ; Liu, 2006; Ma and Wei, 2006; Xu et al., 2007 ; Huang et al., 2008 ; Li et al., 2008 ; Liu et al., 2009 ). Under the influence of human activity and natural forces, contradiction between supply and demand of water resource is increasing substantially, especially at the two downstream oases of Wuwei and Minqin. Due to climate change, obvious runoff reduction of Shiyang River is more than three other inland river basins in Hexi area. In addition, accounting for 88% to 92% of incoming water and groundwater are impounded and exploited at Liangzhou oasis located in the middle and lower reaches of Shiyang River, where rivers and lakes have nearly dried up. Also, the excessive exploitation of groundwater in Minqin has caused the near collapse of the natural ecosystem of the entire oasis (Li and Xiao, 2005; Li et al., 2005 ; Ma and Wei, 2008; Zhang et al., 2011 ; Zhong, 2011). As a "solid reservoir", river runoff can be regulated by glacier mass balance, and researches have pointed out that when glacier coverage in the basin is more than 5%, effect of river regulation is obvious. When glacier coverage is more than 10%, river runoff is basically stable (Ye et al., 1999 , 2012). Related simulation studies also point out, under the background of rising temperatures, change rate of glacier volume is greater than length and area, and runoff of small glaciers are more sensitive to climate change (Ye et al., 2003 , 2005). Thus, glacier observation at upper Shiyang River is very important. In this study, based on topographic maps and remote sensing data, combined with nearby meteorological station data, variation of glaciers and climatic background of the Xiying River Basin was analyzed from 1956/1972 to 2008. This study provides a scientific basis to analysis the current situation and changes of water resources.2 Study area
The Xiying River located in Lenglongling area, East Qilian Mountain, China, is one of eight tributaries of the Shiyang River (Figure 1). Runoff of Xiying River is 38% of Shiyang River (Liu et al., 2013a ). The Xiying River Basin include 4 sub-stream basins, there are Shuiguan River (Basin No. 5Y416G), Ningchan River (Basin No. 5Y416F), Qingyang River (Basin No. 5Y416E) and Luotuo River (Basin No. 5Y416D). There are 42 glaciers in the Xiying River Basin, 15 are towards the North-East (NE), 10 are towards the North (N), 6 are towards the South-West (SW), and 1 towards the West (W) (Figure 2). Thirty-five glaciers are less than 1 km2, and seven are 1~4 km2 (Wang et al., 1981 ). The upstream region of Shiyang River is mainly affected by summer monsoon, with increased precipitation on the windward side. Characteristics of glacier mass circulation is high accumulation and strong ablation, glaciers in this region are sensitive to climate change, and water recharge rate of glacial melt is about 7%~15% (Shen et al., 2001 ). In the Xiying River Basin, average precipitation is about 800 mm (Gao and Yang, 1985), and average snow line which is the lowest in the Qilian Mountains is about 4,450 m a.s.l. (Yang, 1991). Previous observations noted that the distribution of annual precipitation is very uneven, precipitation in second half year accounts for more than 80%, 60% of which appears in Jun–Aug. Precipitation in September to November is usually more than March to May, with day-time precipitation more than night-time in mountainous areas. At present, systematic research and observations have been carried out at Ningchan River Glacier No. 3 and Shuiguan River No. 4 (Li et al., 2010 ; Liu et al., 2013c ).
In this study, two topographic maps have been used. One is an annotated aerial photograph acquired in 1956, which includes the Shuiguan and Ningchan river basins. The second is an annotated aerial photograph acquired in 1972, which includes the Luotuo and Qingyang river basins. Three Landsat images have been used, there are TM (acquired in 1987-9-9 and 1995-6-11), and ETM+ (acquired in 2008-8-9). All the remote sensing images and topographic maps have been corrected to UTM coordinates. The image registration error is less than half a pixel. Color-composite images are composed by three bands of TM image (5, 4, 3). Researchers used artificial vector quantization to extract glacier boundaries (Figure 3). When the slope is less than 25°, the error of 1:50,000 topographic is less than 11 m, when the slope is greater than 25°, the error is less than 19 m (State Bureau ofSurveying and Mapping, 2007). In this study, we compared field surveying data with Landsat image data, the error of artificial vector quantization from Landsat image is about 1.83% (Liu et al., 2013b ), which is acceptable in this study.
Table 1 shows changes in glacier area of Xiying Basin. From 1956/1972 to 1987, the total area of 42 glaciers increased about 5.3%; from 1987 to 1995, glacier area reduced by 12.66% (1.58% per year on average), and one glacier disappeared (Table 2); from 1995 to 2008, total glacier area reduced by 14.97% (1.15% per year on average), and eight glaciers disappeared (Table 2).
In 1956/1972 to 1987, there was a small increase in glacier area of 10.14% (0.68% per year on average) in the Ningchan River Basin, and by about 2% (0.07% per year on average) in Luotuo and Qingyang river basins. Since 1987, retreating of glaciers area in the Luotuo River Basin has accelerated, but in other basins, the retreating trend has weakened to some extent.
Statistics of glacier area changes on altitude zone based on 1956/1972 contour lines shows, from 1956/1972 to 2008, ice cover had already disappeared at 4,000~4,100 m a.s.l.. and decreased by 62% at 4,100~4,200 m a.s.l.. Shrink tendency of ice area is weakening with increasing altitude. At the glacier top (4,800~4,900 m a.s.l.), there is also a certain amount of shrinkage (Figure 4). Statistics on annual variation rate of glaciers with different direction shows reduction in the NE is the fastest and SW is faster (Figure 5).
In 1956~1976, the terminal retreat and area reduction of Shuiguan River glacier No. 4 (Glacier No. 5Y416G4) are 320 m and 0.97% per year respectively; in 1976~1984, 69.7 m and 0.12% respectively; in 1972~2007, 225 m and 0.04 km2 respectively, and at the ablation zone, there is constantly ice thinning (Li et al., 2010 ). Since 1972, the terminal retreat, area reduce and volume decrease of Ningchan River Glacier No. 3 (Glacier No. 5Y416F3) are 6%, 13.1% and 35.3%, respectively, and glacier with progressive thinning (Liu et al., 2013c ). Variation tendency of those two glaciers are the same at a basin scale.5 Discussion
Some studies have pointed out that the temperature increase especially in summer is a decisive reason for ice melting (Shi, 2001). Using temperature and precipitation data at Menyuan and Wushaoling meteorological stations (Figure 6), the rates of temperature and precipitation change were calculated. We found average summer temperatures of Menyuan and Wushaoling have increased by 0.03 °C and 0.02 °C respectively, the rates of annual average temperature at these two locations are same as the rates of average summer temperature, and precipitation of Menyuan and Wushaoling have decreased by 0.4 mm and increased slightly, respectively.
From 1956/1972 to 2008, reduction of glacier area is 20.76%, which is more than other regions, e.g., reduction of Glacier No. 12 in Laohugou Valley located in west Qilian Mountain is lower than Ningchan River Glacier No. 3, Shuiguan River No. 4 is 4% in 1958~2009 (Liu et al., 2011 ; Liu et al., 2013c ), Tuotuo River Basin is 3.2% in the late 1960s (Zhang et al., 2008 ), western Nyainqentanglha Range is 6.1%±3% between 1976 and 2001 (Shangguan et al., 2008 ; Bolch et al., 2010 ) and Gongga Mountain region is 11.3% from 1966 to 2009 (Pan et al., 2011 ). In recent decades, glaciers in the Xiying River Basin are shrinking seriously. Hence, glaciers in the Xiying River Basin are very sensitive to climate change.6 Conclusions
From 1956/1972 to 2008, in the Xiying River Basin, glacier area reduced by 20.76%, but increased slightly around 1987, nine glaciers have disappeared, and reduction tendency of glaciers are accelerating. Because of glacier shrinkage, there is no ice cover below 4,100 m a.s.l.. Retreat rate of glaciers is obviously controlled by glacier orientation. Temperature increase especially after 1987 is the main reason for glacier shrinkage. Glaciers in the Xiying River Basin are very sensitive to climate change. In this study, Landsat TM/ETM+ is the data source used to extract glacier boundary, cloud and snow cover are critical factor of image interpretation. Thus, other observation method is needed in further work.Acknowledgments:
This study was supported by the Project from the National Foundational Scientific and Technological Work Programs of the Ministry of Science and Technology of China (Grant No. 2013FY111400), the State Key Laboratory of Cryospheric Sciences (SKLCS-ZZ-2017), the Global Change Research Program of China (2013CBA01801), the Chinese Academy of Sciences (KJZD-EW-G03-01) and the National Natural Science Foundation of China (41371091), we also thank Junli Xu provide the Coordinate system conversion system v1.0 (2016SR394422).
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