The changing water cycle of the mountains

Much of the freshwater sustaining human societies is generated in the mountains: the mountain cryosphere supports almost a third of the global population for irrigation, drinking water, industry and the environment. At the same time, this crucial resource is undergoing unprecedented changes, with glaciers shrinking, snow decreasing globally and permafrost thawing across continents. This bigger picture masks a very large variability of responses across climates and continents, shaped by processes specific to different mountain ranges. Glaciers and seasonal snow are often assumed to respond to a changing climate in a linear manner, especially at large and global scales, given the complexity of interactions among them and the eco-hydrology of the catchments they sustain. Growing evidence suggests more complex dynamics and threshold effects that will affect the water resources they generate. 

In this talk, I will focus on processes and non-linearities in the mountain cryosphere that shape the mountain water cycles across climates, and show how that water cycle is changing  across regions as a result. I will focus on a number of specific processes: i) the role of ephemeral and marginal snowpacks on streamflow generation, and their vulnerability to temperature and precipitation shifts, especially in sub-tropical regions; ii) changes in precipitation phase, and their distinct effects on the water cycle depending on precipitation seasonality; iii) the transition from sublimation to melt in a warmer world and how that can change the assumed linear trajectory of water from glaciers and snow in arid areas; iv) the role of evaporative fluxes in the mountain water cycle and how warming promotes increased evapotranspiration that recycle increasing portions of high-altitude precipitation and surface water into the atmosphere. Another key disruption in the functioning of mountain systems is the increasing frequency and intensity of droughts, and I will show some very recent results about how glaciers buffer droughts, and how this capacity might be hampered when droughts become more severe and of longer duration. We use for most of these investigations a fully mechanistic, physically-based modelling framework that represent both the cryosphere, the biosphere and hydrosphere of mountain regions, and I will also briefly touch on the modelling strengths and limitations. 

Our results show that the response of the cryosphere to ongoing changes in the climate is very heterogenous. Ephemeral snow in sub-tropical, semi-arid climates has been mostly neglected in modelling assessments, invisible to satellite images, but represents the main contributor to water runoff, and yet this will change in the future with increasing temperatures, which will remove a major source of water. Overall, shifting snowline altitudes and shrinking accumulation areas will move the areas of water generation to higher elevations, altering storage and routing patterns and seasonality, and accelerating the water cycle. Droughts are changing the functioning of mountain systems, with evapotranspiration amplifying water deficits in many mountain regions, while snow droughts enhance this so-called drought paradox. I will conclude with a perspective of future research on mountain processes and water resources.