Recent climate and hydrological changes in the Tianshan Mountains, Central Asia

Global warming accelerates the water cycle worldwide, and directly affects hydrological changes and may cause changes in water availability. The Tianshan Mountains, known as “water tower of Central Asia”, is situated in the Eurasia hinterland. It serves as the main water source and ecological barrier in Central Asia. Most rives originated from the Tianshan Mountains are recharged with rainfall, glacier melt and snow meltwater. The hydrological processes in the Tianshan Mountains are strongly affected by changes in temperature and precipitation, as well as changes in the snow and glaciers. Increases in temperature have important consequences for the hydrological cycle, particularly in areas dominated by glacier and snow melt.

This study systematically investigated precipitation and temperature changes and their impacts on glaciers, snow cover and hydrological processes in the Tianshan Mountains using station observations, remote sensing data and reanalysis data. In a warming climate, precipitation is more likely to occur as rainfall rather than snowfall. Temperature-induced precipitation shifted from snow to rain since mid-1990s, with S/P experiencing an overall declining trend at a rate of 0.5%/decade. In addition, an overall increase in extreme precipitation was detected, as reflected in 25 indices. The number of consecutive dry days decreased from 87.02 to 69.35 while the number of consecutive wet days increased from 3.89 to 4.61. Changes in extreme precipitation frequency were shown to increase with event rareness. For R95p, the observed changes in frequency are 34.46%, but these jump to 96.58% for R99p.  By creating a long-term, high-quality, daily snow cover extent (HMASCE) product (1982–2019, spatial resolution of 5 km), the spatial and temporal variability in snow metrics (snow cover area and snow cover phenology) has investigated. Snow cover in the Tian Shan region showed a slight increase during this period, mainly in West Tianshan (0.66% a^-1), Hissar Alay (0.64% a^-1), and East Tianshan (0.24% a^-1).

Approximately 97.52% of glaciers in the Tianshan Mountains showed a retreating trend. For the northern TianShan Mountains,  total area and volume of glaciers exhibited negative trends, decreased by 456.43 km² (16.08%) and 26.14 km³ (16.38%), respectively, from 1990 to 2015. The reduction in the glacier area exhibited an accelerating trend, with a decreasing rate of 0.60% a^-1 before 2000, but of 0.71% a^-1 after 2000. River runoff responds in a complex way to changes in climate and cryosphere. For example, the runoffs of the Kaidu River and the Aksu River, located in the south flank of the Tianshan Mountains, have increased by 27.4% and 14.4%, respectively, during 1960 to 2021. The total water storage in the Tianshan Mountains also experienced a significant decreasing trend with a rate of 12.12 mm a^-1 during 2020~2021..

This study sheds light on current and future changes in water cycle under global warming in the Tianshan Mountains. More efforts should be made on the interpretation of impacts and mechanisms of these changes on runoff, which is a key factor that controls the amount and seasonality of freshwater resources for domestic and agricultural needs.