Abstract
Although evidence of hydrological responses to climate is abundant, changes in water yield (WY) in mountainous regions due to climate change and intensified cryospheric melt remain unclear, mainly because of limited observations and large uncertainties in cryosphere-hydrological modeling. In this study, we used annual runoff observations and a high-resolution precipitation dataset to examine the long-term changes in WY in the Upper Brahmaputra River (UBR) basin, as represented by six sub-basins from the stream head to downstream. We found that WY generally increased during 1982–2013, but regime shifts were detected in the late 1990s. Moreover, the direction of the changes in WY reversed from increasing to decreasing in recent years despite the magnitude of the changes continually increasing from less than 10 % to 80.5 %. Furthermore, we used the double mass curve technique to assess the effects of climate, vegetation, and the cryosphere on WY. The results showed that the climate and cryosphere together contributed to over 80 % of the magnitude increases in WY over the entire UBR basin. However, the combined effects were either offsetting or additive, further leading to slight or substantial magnitude increases, respectively, in which the role of vegetation was nearly negligible. Nevertheless, we found that meltwater from the cryosphere had the potential to alleviate the loss of water availability, which mainly resulted from reduced effective precipitation in most regions. Therefore, the combined effects of climate and cryosphere changes should be considered in ecological restoration and water resources management, particularly involving co-benefits for upstream and downstream regions.
Highlights
Water yield (WY) in mountains is crucial for sustaining fragile ecosystems in the headwaters, supplying valuable freshwater resources to downstream lowlands, and balancing co-benefits between the upstream and downstream areas, especially for large transboundary river systems (Viviroli et al, 2011)
Our results indicated that the climate and cryosphere were important factors for magnitude increases in WY throughout the Upper Brahmaputra River (UBR) basin, but their relative contribution varies across regions
Regime shifts in WY were detected during the late 1990s over the UBR basin
Summary
Water yield (WY) in mountains is crucial for sustaining fragile ecosystems in the headwaters, supplying valuable freshwater resources to downstream lowlands, and balancing co-benefits between the upstream and downstream areas, especially for large transboundary river systems (Viviroli et al, 2011). Changes in WY have been commonly, but separately, attributed to climate changes (Dierauer et al, 2018; Song et al, 2021), vegetation (Goulden and Bales, 2014; Zhou et al, 2021), and the cryosphere (such as glacial snow melt; see Kraaijenbrink et al 2021). These changes are expected to alter the spatial and temporal distribution of water resources (Tang et al, 2019) and further threaten the water supply and food security downstream (Biemans et al, 2019). Changes in the climate, vegetation, and cryosphere have significantly affected the WY over the QTP (Bibi et al, 2018)
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