Assessing the Impact of Deep Subsurface Storage and Flows on Hydrological Processes and Water Availability in Mountainous Regions

In times of climatic unpredictability driven by a quickly changing climate, it is critical to investigate hydrological processes and water availability in different climatic and geomorphological contexts. Mountains have long been acknowledged as fundamental sources of abundant high-quality water for the densely populated downstream areas. The large volumes of water stored in mountain lakes, reservoirs, and snow caps are extremely important to buffer precipitation variability and sustain ecological and anthropic water uses during droughts. So far, the flow and storage of water in the deeply fractured rock formations constituting the core of mountain massifs have mostly been neglected, even for the long-term water balance. However, recent experimental evidence has shown that poorly porous and conductive fractured bedrock can host aquifers whose contribution to streamflow can be substantial, particularly during droughts.

This study systematically assesses, under a wide range of geomorphoclimatic conditions, how deep subsurface storage and flows affect critical hydrological and hydrogeological variables such as the age of streamflow (as opposed to the age of baseflow), surface seepage, and permanent drainage density. These critical hydrological processes are investigated via a large set of steady-state numerical experiments by modulating surface topography, groundwater recharge, and hydrogeological properties of the subsurface (e.g., formation depth, hydraulic conductivity, and its heterogeneity).

The results quantitatively show, for example, how different morphological and hydrogeological conditions may respond to climate change and can be useful in identifying vulnerable areas where mitigation strategies should be prioritized to cope with water shortages. The study can also help understand where ecological alterations driven by the lack of water can have a more profound impact on riverine habitats and where to expect the shift of species in the future.