Glacial runoff likely reached peak in the mountainous areas of the Shiyang River Basin, China

Glacier runoff in mountain areas of the Shiyang River Basin (SRB), Qilian Mountain, western China is important for the river and water supply downstream. Small glaciers with area of less than 1 km(2) are dominant (87%) in the SRB. A modified monthly degree-day model was applied to quantify the glacier mass balance, area, and changes in glacier runoff in the SRB during 1961-2050. The comparison between the simulated and observed snow line altitude, annual glacier runoff, and mass balance from 1961 to 2008 suggests that the degree-day model may be used to analyze the long-term change of glacier mass balance and runoff in the SRB. The glacier accumulation shows a significant (p < 0.01) decreasing trend of -0.830 mm a(-1). The mass balance also shows a significant (p < 0.01) decreasing trend of -5.521 mm a(-1). The glacier total runoff has significantly (p < 0.05) increased by 0.079 x 10(5) m(3) from 1961 to 2008. The monthly precipitation and air temperature are projected to significant (p < 0.005) increase during 2015 to 2050 under three different scenarios. The ablation is projected to significant (p < 0.001) increase, while the accumulation has no significant (p=0.05) trend. The mass balance is projected to decrease, the glacier area is projected to decrease, and the glacier runoff depth is projected to increase. However, the glacier total runoff is projected to decrease. These results indicate that the glacier total runoff over glacier areas observed in 1970 reached its peak in the 2000s. This will exacerbate the contradiction between water supply and downstream water demands in the SRB.