Impact of transient groundwater storage on the discharge of 1 Himalayan rivers 2 3

16 17 The transfer of precipitation into rivers involves temporary water storage in reservoirs 1,2 such as 18 soils, groundwater, snow and glaciers, where different residence times influence the hydrological 19 cycle. In the central Himalayas, the water budget is considered to be primarily controlled by 20 monsoon rainfall, snow and glacier melt 3,4 , and secondarily by evapotranspiration 3. The 21 existence of a deep groundwater contribution 5-7 is deduced from the chemistry of Himalayan 22 rivers 6. However, its importance in the annual water budget remains to be evaluated. Here, we 23 analyze ~30 years of daily precipitation and discharge within major catchments in Nepal. We 24 observe annual precipitation-discharge hysteresis loops, in both glaciated and unglaciated 25 catchments, independently of the geological setting. This implies the temporal storage of water in a reservoir whose characteristic response time (~45 days) represents a typical diffusivity (~1 m² s-27 1) of fractured basement aquifers 8. This transient storage capacity is of ~28 km³ for the three 28 main catchments of Nepal, whereas we estimate snow and glacier melt contribution to be ~14 29 km³ yr-1 (~10% of the annual river discharge). We conclude that groundwater storage in 30 fractured basement constitutes an important compartment of the Himalayan river discharge 31 cycle, that can be quantified through the study of precipitation and discharge throughout the 32 year. 33 34 The discharge of the central Himalayan rivers is governed by a strong precipitation seasonality 35 3,6,9,10 (Fig. 1) with up to 80% of the annual rainfall occurring during the Indian Summer Monsoon 36 (ISM) season 3. The ISM precipitation is the main source for glacier mass accumulation 9 and its spatial 37 distribution is strongly influenced by orographic effects 3. Variations in intensity and duration of the 38 ISM, linked to El Nino/Southern Oscillation (ENSO) 11 , enhance the annual amount of precipitation by 39 ~25 to 50% with respect to the annual mean at low to moderate elevation (>3 km), and up to 200% at 40 high elevation 12. Snowmelt contributes to a significant fraction of river discharge in the western and 41 eastern Himalayas and on the Tibetan plateau 3,13 , but only to a minor fraction (~10%) in the central 42 Himalayas, mainly in the early ISM (May to July) 3. It has been suggested that rainfall-derived 43 discharge, ice and snowmelt are the primary factors controlling Himalayan river …