Global glacier contribution to streamflow

Glacier contribution to streamflow has mostly been investigated at the scale of relatively small catchments, and more rarely at the scale of the major rivers. In this study, we compare monthly glacier mass changes to monthly estimates of streamflow for the period 1990-2023, both annually and seasonally for 55 major river basins larger than 5700 km² (0.01 to 20.0 % glaciated). Monthly mass changes for every individual glacier are obtained by temporally downscaling geodetic elevation change observations with the variability from in situ glaciological measurements and global-scale model results. Streamflow is based on GLOFAS and G-RUN global datasets. GLOFAS is a land surface model forced by ERA5 reanalysis that feeds a channel routing model. G-RUN is a machine learning algorithm that predicts monthly runoff based on the Global Soil Wetness Project Phase 3 dataset. Basin scale precipitation and evapotranspiration are estimated from ERA5 reanalysis data.

Annual glacier mass change and thus water release ranges from near zero to 550 mm at the basin-scale and roughly correlates with the percentage of the glacierized area in the basin.  The ratio of annual glacier mass change divided by the mean annual discharge, hereafter called the annual glacier contribution, is below 6 % for all the basins larger than 500’000 km², with the exception of the Indus river with an annual glacier contribution of 25 % (40 mm). The Indus river basin is both highly glacierized (more than 3 %) and arid, explaining such a high ratio.

The glacier seasonal contributions, defined as the water volume derived from glacier mass change during summer months (JJAS in the northern hemisphere and DJFM in the southern hemisphere), divided by the mean discharge in the same months, are always higher than the annual ones. In particular for the basins with low flow during the melt season (e.g. Rapel, Skagit, Rhone, Columbia, Biobio, Po, Rhine), the seasonal contribution is more than three times the annual one. On average 62 % of the glacier mass change originate as a balance contribution, meaning that it corresponds to the seasonal snow accumulation. In contrast, 38 % of the glacier contribution originates from ice melt, i.e. the unsustainable release of the solid water stored in glaciers.

Besides uncertainties in glacier mass change and streamflow data, evaporation (of the glacier meltwater) and groundwater contributions are not treated explicitly, which might lead to overestimations of the glacier contributions. It should thus be seen as a first-order estimate that highlights the major contrasts between basins at the global scale.