Abstract: The Yarlung Zangbo River is a river of rich water resources, but its upper reach runoffs are impacted significantly by climate changes, glacier and frozen soil degradation. This paper develops a distributed eco hydrological model (GBEHM) coupled with cryospheric process modeling to simulate the runoff changes in its upper basin, focusing on an analysis of the variation trends of the hydrological elements in the study area and a quantitative assessment of the impact of climate changes. The results show that from 1981 to 2010, the annual runoff and evapotranspiration in this basin experienced a significant increase, and precipitation increase contributed most to the runoff increase. This period saw a 7% decrease in the permafrost area, a 30.6 cm/10a increase in the thickness of the permafrost active layer, and a 7.3 cm/10a decrease in the annual maximum freezing depth of seasonal frozen ground. The water reserves in the glaciers were decreased significantly at a rate of 1 billion m3/a, while their melting runoff increased at 2.7 mm/a.

Key words: runoff change, cryosphere degradation, distributed hydrological model, upper Yarlung Zangbo River basin