Hydrological processes for rivers fed predominantly by snow and glacier meltwater will be more complex under global warming scenario. Tienshan Mountains, "water tower of Central Asia", provide the source of many transborder rivers that bring fresh water to more than 100 millions of people. Researchers revealed the recent changes in water discharge in snow and glacier melt-dominated rivers in the Tienshan Mountains, Central Asia, which is of great significance for understanding and mastering the future changes in water resources and improving the management level.
Researchers from the State Key Laboratory of Desert and Oasis Ecology, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, give a systematic analysis of runoff changes in these two contrasting headwater rivers-Toxkan River (with glacierized area of 3.66%) and Kumalak River (with glacierized area of 16.34%) from 1990 to 2015, and a comprehensive analysis was adopted to estimate potential reasons for their changes. These include temperature, precipitation, and altitude of the 0 ℃ isotherm, snow cover area and snow depth, and glacier mass balance according to statistical mechanics and the maximum entropy principle (SMMEP) model and the sensitivity model.
In addition, they also discuss the impacts of these drivers on hydrological processes in different spatial and temporal, look for potential links, and examine responses to the drivers' variabilities.
The results show that, for the Toxkan River catchment with a small glacierized area during 1979-2015, to date, the steady increase in temperatures has not only decreased the snow cover area and snow depth during wintertime and springtime. The annual number of snow cover days has also decreased, along with the maximum runoff in spring and winter has advanced, accompanied by a significant rise in snowmelt runoff and a reduction in summertime runoff. In contrast, the summer runoff in the Kumalak River showed a sharply decrease.
From 1979 to 2002, both the runoffs of the Toxkan and Kumalak Rivers exhibited a strongly positive trend at rates of 0.57 × 108 m3/a and 0.66 × 108 m3/a, respectively, the upward trend was reversed to downward at rates of 1.02 × 108 m3/a, post 2002. For the Toxkan River, the increased spring and summer runoff during 1979-2002 was mainly caused by the positive trends in spring snow meltwater and summer precipitation, whereas the decreased spring runoff since 2002 was directly related to the significant decrease in spring precipitation. The reduction in summer runoff resulted from the decrease in snow meltwater, owning to the earlier snowmelt period during spring as well as to the sharp shrinkage in the glacier area in the catchment.
For the Kumalak River, increasing temperature with a related increase in glacier meltwater was the dominant driver for the summer runoff increase during 1979–2002. However, the substantial decrease in summer runoff since 2002 was related to the drop in summertime temperatures (both surface and upper-air temperatures). Summer temperature reductions accompany with increased summer snow cover area and snow depth at higher elevations (3500-4500m) where glaciers develop, have slowed the melting of glaciers, with mean annual glacier mass balance decreasing from -0.57m to 0.37m.
The results entitled "Recent Changes in Water Discharge in Snow and Glacier Melt-Dominated Rivers in the Tienshan Mountains, Central Asia", were published in the journal of Remote Sensing.
Article lilnk: https://www.mdpi.com/2072-4292/12/17/2704/htm
Schematic diagram of the combined effects of temperature, precipitation, SCA, SD and GBM changes on Toxkan and Kumalak River runoff in the Central Tienshan Mountains during 1979-2002 and 2002-2015