Climate-driven changes in hydrological and hydrodynamic responses in the Yarlung Tsangpo River

Abstract

The High Asia region supplies a substantial part of the water demands in South Asia, playing an important role in maintaining ecosystem stability both locally and downstream. The Yarlung Tsangpo River (the upper Brahmaputra) basin in High Asia is especially sensitive to climate change; however, climate-driven hydrological changes and hydrodynamic processes are not well understood. In this study, hydrological and 2-D hydrodynamic numerical modelling were combined, providing a better representation of basin runoff and wetland fluvial dynamics with improved spatial resolution over the sparsely gauged Yarlung Tsangpo River, and the combination provided reliable results for watershed and wetland analysis. From 1979 to 2019, significant upward trends (p < 0.05) in temperature were observed in Lazi, Zedang, Linzhi, and Bomi, located from the upper reaches to lower reaches. However, the variation trends of precipitation displayed great spatial differences. As a result, runoff in Lazi, Zedang, and Bomi showed an increasing trend, with the most significant trend observed in Zedang (p = 0.04). With the increases in wetland area and water depth, but non-significant variations in velocity, the impacts of climate variations were more pronounced in rainy periods than in snowmelt periods. During rainy periods, the water depth in the western wetland ranged from 0 m to 7 m in the early 1980s and middle 2000s and increased to 10 m in the late 2010s. Furthermore, our study revealed that changes in the daily maximum temperature, the daily minimum temperature, and the ratio of snow period precipitation to rainy period precipitation accounted for most of the variance observed in rivers and wetlands. In particular, the upper reach, with more obvious freeze–thaw processes, was more sensitive to temperature changes than were the middle to lower reaches during snowmelt periods. This study provides vital information for the sustainability of water resource management and wetland conservation in the context of climate change in the High Asia region.