Increased occurrence of ephemeral snowpacks will augment groundwater recharge and worsen summer low flow conditions in the Alps

In snow-influenced catchments around the world, snowmelt is more efficient at recharging groundwater than rainfall. As climate warms, more precipitation is expected to fall in the form of liquid rain than as solid snow. The future accumulated snowpacks will melt earlier and more often, resulting in shallower snowpacks that are more ephemeral, i.e. such snowpacks will accumulate and melt during winter. This contrasts with current seasonal snowpacks that accumulate during winter but melt during spring. The impact of this snow regime shift on the quantity of groundwater recharge is largely unknown. <P />Using stable water isotopes and baseflow recession analysis over 39 headwater catchments in Switzerland, we show that ephemeral snowpacks are more efficient at recharging groundwater than seasonal ones. This is due to higher winter melt frequency and intensity in catchments characterized by ephemeral snowpacks. We also observe a strong elevational divide currently in Switzerland at 1500 m a.s.l., below which catchments are mostly characterized by ephemeral snowpacks. This elevational divide will shift to around 2000 m a.s.l. with a 2.5°C atmospheric temperature rise, increasing the total amount of groundwater recharge in catchments between 1500 and 2000 m a.s.l. Even with largely unchanged annual precipitation, we demonstrate that the increased winter groundwater recharge will translate into lower summer flows, increasing the likelihood of summer droughts. Our study has important implications for flow regimes and water resources in the Swiss Alps, that are likely transferable to other mountainous regions in the world.