The glacier geodetic mass balance and its hydrological contribution to runoff in the Yarlung Zangbo-Brahmaputra basin

The High Mountain Asia (HMA) hosts the most significant number of glaciers outside the polar region, called the "Third Pole."  Glacier meltwater is vital for 1.5 billion inhabitants in the HMA. The Yarlung Zangbo–Brahmaputra (YB), a transboundary river flowing from China through India and Bangladesh into the Bay of Bengal supporting more than 80 million people, is fed by over 15,000 pieces of glaciers. However, glacier melt contribution to the runoff in the YB by sub–basins is still unclear. In this study, we presented an updated glacier inventory for debris–free glaciers in the YB for 2020, calculated geodetic mass balance by glacier surface elevation difference, using NASADEM and GLO30 DEMs from 2000 to 2013, laser altimetry data from ICESat–2 from 2018 to 2023, and available elevation difference datasets from 2000 to 2020. Additionally, we studied nine individual glaciers to verify our geodetic glacier mass balance results with available in–situ observations and to better understand the glacier dynamics in the basin. Our study shows that (1) the glacier area decreased from 12,638.3±758.3 km² in the 1970s to 9,081±12.09 km² in 2020, with a loss of 28.15% of debris–free glacier area at –0.56 % a^–1 in the past fifty years; (2) glacier mass balance (GMB) was –0.49 ± 0.02 m w.e.a^–1 from 2000 to 2013, with glacier mass change (GMC) at –4.61 ± 0.41 Gt a^–1; (3) based on ICESat–2 and GLO30 from 2013 to 2023, the GMB was –0.47 ± 0.02 m w.e.a^–1, and GMC was –4.44 ± 0.38 Gt a^–1; (4) the GMB was consistently negative in the YB from 2000 to 2020 at five–year intervals, with –0.36 m w.e.a^–1 (–3.95 Gt a^–1) in 2000 – 2005, and –0.64 m w.e.a^–1 (–7.07 Gt a^–1) in 2015 – 2020; (5) the GMB contribution to runoff increased from 0.05 % a^–1 during 2000 – 2005 to 0.86 % a^–1 during 2015 – 2020, with an average of 0.59% a^–1 in the YB from 2000 to 2023.