Morphological and Hydrogeological Controls of Groundwater Flows and Water Age Distribution in Mountain Aquifers and Streams

Mountains are an essential source of the terrestrial component of the hydrological cycle, supplying high-quality water to river networks and floodplain aquifers, especially during droughts. Traditionally, mountain hydrology has focused on shallow processes, overlooking the significance of deep-seated rock formations due to characterization challenges. Recent field studies have revealed that fractured rock formations can host rich aquifers despite their low permeability. Nonetheless, it is unclear how deep flows interact with the overall hydrological functioning of mountain areas, how they contribute to the long-term water budget, and how climate, morphology, and geology jointly control them. Through numerical simulations, we have gained new insights into mountain aquifers, addressing (a) the proportion of groundwater base flow and its age distribution, (b) water storage and its sensitivity to groundwater recharge, (c) the impact of long term mean recharge on the extent of the groundwater-fed surface drainage network under various morphological and geological settings. We showed that subsurface travel times follow a Gamma distribution, whose parameters are modulated by recharge, hydraulic conductivity, and topography. High recharge and strong decay with depth of the hydraulic conductivity in a hilly topography lead to a shallow water table mimicking the surface topography and spatially distributed low-intensity outflows that feed a dense drainage network. In rugged catchments, the groundwater contribution intensifies and concentrates in the downstream portion of the river network as recharge declines. These findings can help assess how a changing climate might impact hydrological regimes under various geomorphological conditions and identify sustainable water uses in mountain environments.