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A Primary Study of the Relationship Between Glacial Mass Balance and Climate in the "July First" Glacier

Liu Chaohai and Xie zichu (Lanzhou Institute of Glaciology and Geocryology, Chinese Academy of Sciences)  
The average summer (from June to August)air temperature is 1℃lower and zero-equilibrium line 88 m lower in the 18 years after 1968 than that in the 11 years before 1968 in the middle part of Qilian Mountains. It was found from the mass balance reconstructed according to the air temperature and precipitation from 1957 to 1985 on the"July First" Glacier that the glacier was characterized by the negative mass balance state with a cumulative negative mass balance value of 232.9 × 104m and by the possitive mass balance state with a cumulative possitive mass balance value of 773.4×104m3 in the 18 years after 1968. In the recent 30 years, the net mass balance has increased by 540.5×104m3 and average glacial thickness by 1.8m. Relatively larger possitive mass balance appeared in 1967/1968, 1975/1976, 1982/1983, which showed the cycle of 7 - 8 years. The fluctuation of 1 m of the zero-equilibrium line corresponds to the mass balance change of 0.68×104 m3. If the temperature above and below zero ℃at the glacial terminus is taken as the temperature index for the beginning and end of the glacial melting period respectively, the melting period is averagely 14 days shorter in the 18 years after 1968 than in the 11 years before 1968 and the maximum ablation altitude has droped for 130 m in average on the "July First" Glacier. Glaciers in the Qilian Mountains are typical glaciers accumulating in summer which possess following features: c cw, a aw; b' 0 , b'w 0 (accumulation area);6" 0, b"w0 (ablation area). On such kind of glaciers which are typical continental glaciers,mass bal~ ance processes are mild and the level of mass balance low. The shortening of the ablation period and drop in summer temperature since mid 1960s and the increase in precipitation since 1970s have resulted in the possitive mass balance and decrease of glacial retreat.
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