Lake water and glacier mass gains in the northwestern Tibetan Plateau observed from multi-sensor remote sensing data: Implication of an enhanced hydrological cycle

Lakes and glaciers are widely distributed in the northwestern Tibetan Plateau. A synchronized examination of lake and glacier mass variations there has not previously been conducted, which has limited the understanding of the lake water balance and the hydrologic cycle in the region. In this study, we comprehensively examine lake area and volume, and glacier mass changes using multi-sensor satellite data. We find that lake expansion in the northwestern Tibetan Plateau was more robust from 1976 to 2018 when compared to other regions of the plateau, especially for the glacier-fed and endorheic lakes. Lake volume changes show that most of the lakes had an increase in water volume, particularly in 20002018 with a total water storage gain of similar to 28.6 Gt. By using high-resolution KH-9 and TanDEM-X data, we observe that the glacier surface elevation of the western Kunlun Mountains had a slight thinning rate of -0.07 +/- 0.16 m/yr in 1973-2000, but a positive rate of 0.002 +/- 0.003 m/yr in 2000-2018. The heterogeneous pattern of glacier elevation changes between the north (N) and south (S) slopes are revealed, i.e. -0.02 +/- 0.01 m/yr (N) against -0.12 +/- 0.03 m/yr (S) in 1973-2000 and - 0.05 +/- 0.02 m/yr (N) against 0.06 +/- 0.02 m/yr (S) in 2000-2018. Overall, the glaciers trend to a stable state (similar to 0.05 m/yr) in both the south slope of the western Kunlun Mountains and Aru Co regions between 2000 and 2018. Similar patterns are also found for basin-wide examinations of lake storage changes (mass gains) and glacier mass budgets (positive or close to stable). The seasonal snow cover area changes, derived from cloud-free MODIS snow cover products, present a variable and insignificant trend between 2003 and 2017. Snow depth derived from passive microwave remote-sensing data, exhibits a decreasing trend between 1979 and 2015, but the water equivalent could contribute only an insignificant amount to the observed lake changes. The lake water gains, and almost positive glacier mass balance imply that the hydrological cycle in the northwestern Tibetan Plateau has become enhanced.