In the course of the transfer of precipitation into rivers, water is temporarily stored in reservoirs with different residence times1, 2 such as soils, groundwater, snow and glaciers. In the central Himalaya, the water budget is thought to be primarily controlled by monsoon rainfall, snow and glacier melt3, 4, and secondarily by evapotranspiration3. An additional contribution from deep groundwater5, 6, 7 has been deduced from the chemistry of Himalayan rivers6, but its importance in the annual water budget remains to be evaluated. Here we analyse records of daily precipitation and discharge within twelve catchments in Nepal over about 30 years. We observe annual hysteresis loops—that is, a time lag between precipitation and discharge—in both glaciated and unglaciated catchments and independent of the geological setting. We infer that water is stored temporarily in a reservoir with characteristic response time of about 45 days, suggesting a diffusivity typical of fractured basement aquifers8. We estimate this transient storage capacity at about 28 km3 for the three main Nepal catchments; snow and glacier melt contribute around 14 km3 yr−1, about 10% of the annual river discharge. We conclude that groundwater storage in a fractured basement influences significantly the Himalayan river discharge cycle.
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