What are the challenges and the climatic particularities in the mountains?

This article echoes Dr. Eeckman's presentation "Climate Variability in the Mountains - Issues and Features" from the CLIMACT Seminar Series.

JE: I did my doctoral thesis on watersheds in the Himalayas and I am currently working on the Alps. The problems are quite similar, in both cases I am doing hydrology on rivers at the local scale. This allowed me to present my two fields of study in parallel.

My research problematic concerns the risk of drought in the canton Vaud Alps, and in particular the infiltration of snow melt which partly conditions the availability of water in low water periods. This winter's snow melted very early and this is a real issue. On the one hand, there is less and less snow and on the other hand, it is melting earlier, which disrupts the whole hydrological cycle.

JE: Since I'm still working on the data analysis, I can't give a conclusion yet. At the same time, in research, concluding often means coming to a new question. The main challenge of my research now is to quantify the time of water storage in the soil. In general, in the mountains, the soils are very thin (maximum 30-40 cm) and very steep. In hydrological models, it is usually assumed that rain and melt water mainly run off the surface rather than infiltrate into the soil. Infiltration into mountain soils is not well modeled. Mountain subsoils can be very heterogeneous and chaotic former landslides or very old glacier moraines on which soils have developed. Inside these, there are macro-pores (huge holes) that can store a lot of water. At the end of my thesis, I estimated with a lot of uncertainty that there can be up to three to four months of storage in the soil, for small basins. But in the Himalayas, the climatology is different from our regions because of the monsoons. This storage time value cannot be directly applied to catchments in Switzerland.

These storages are sorts of reserve and could get us out of difficult situations. The problem at the moment is that the snow melt occurs very early, so that the river levels are already low at the beginning of the summer. These low water levels (very low water level in the rivers) are further accentuated by the lack of rainfall and high temperatures during the summer, like we are experiencing this year.

The storage of meltwater in the soils, which gradually leaks into the rivers, can be our "survival card" to ensure minimum levels in summer. But to know if we can really rely on them, we need to study them to fully understand how they work.

JE: It's a period when there is low water availability and simultaneously high demand, which creates stress. The high demand can come from vegetation (high demand for water in the spring with leaf growth, for example) or from human needs, especially for agriculture and tourism. The end of summer can be a period of water stress in the West because it is a low water period.

Having low water periods is a principle of the annual cycle, even trees are adapted to this. In autumn, they take advantage of the fact that they reject the water they contain into the ground to avoid freezing during the winter.

With global warming, this phenomenon is intensified, because there is less water available during low water periods, either in the soil or in rivers. The demand is the same for vegetation. But summers are hotter, meaning more evaporation and the trees will draw even more. There will be less water overall in the summer (this may vary locally) and there will be more evaporation. As far as human demand is concerned, there are more and more of us, and it is exponential. The demand is growing.

The consequence of too much withdrawal compared to availability is a lack of water and this can lead to droughts. There are three quite different types of droughts : climatological, hydrological and agronomical. Droughts that cause water to dry up in rivers have several very direct impacts on river withdrawals (drinking water or water for agriculture) but also on hydropower. The droughts that cause water to dry up in the soil are related to vegetation, because vegetation takes a lot of its water supply from the soil, especially in the absence of rainfall.

JE: The periods of stress in the Alps and in the Himalayas are completely different. In the Alps, it takes place more in late summer, early autumn. In the Himalayas, it's more at the end of winter. This is not due to the temperature, but to the fact that these are different dynamics. The winters are very dry in the Himalayas while the summer is humid. In the West, we have winters that are generally more humid. At the end of winter in the Himalayas, if the thaw doesn't come or doesn't come at the right time, there is no more water in the soil. The thaw and melt will balance out the fact that there has been no rain for several months.

In Nepal, there is also mass tourism. They are often Westerners used to having plenty of water and taking hot showers for tens of minutes for example. The months of April and May are very touristy because they are the mildest periods. However, these are the periods when there is the least water because it is the end of winter. There has been no precipitation for several months and the fact that the thaw occurs at the same time as tourism is essential.

JE: These two diagrams are from the IPCC's sixth report (published on April 4, 2022). They show the evolution of monthly temperatures since 1950 for two large geographical regions defined in the report: the Tibetan plateau and "central and western" Europe. Each line corresponds to a month, and the columns are years. We observe that the diagrams are increasingly red, which means that monthly temperatures are increasing globally.

For the European mountains, the warming intensifies in summer. We have more and more important and frequent heat waves.

On the Tibetan plateau, the warming does not occur in summer because of the monsoon. The rains counterbalance the warming. But it is in winter that the climate warms up a lot. Winters are getting warmer and warmer in Nepal. But the common point between these two regions is that we observe a significant warming of the first months of the year (January to March), which are, as we have seen before, the crucial months for snow melting.

JE: In Switzerland, there is an overall decrease in rainfall in summer. Switzerland is quite "lucky" because it is not the place most affected by summer droughts. But MeteoSwiss does show trends of decreasing rainfall in summer, resulting in summer drought.

In Nepal and the regions affected by the Asian monsoons, there is an increase in the intensity of the monsoon: more rain in a shorter period. This will greatly disrupt the hydrological cycle resulting in very heavy rainfall. This will increase the risk of floods, landslides and debris flows and consequently the water stress effect explained above. In March-April, rain is expected and ideally, from May onwards, it should start raining little by little to water the fields, fill the rivers and build up stocks. This is not what will happen in the future: there is a period of water stress that will extend until the arrival of the monsoon.

About the author:

Judith Eeckman is a post-doctoral researcher in hydro-climatology at the Institute of Geography and Sustainability (IGD) of the University of Lausanne. Dr. Eeckman holds an engineering degree in applied mathematics and computer science and a Master's degree in physical geography.

As a first assistant at the IGD, Dr. Eeckman is conducting research on the characterization of drought risk at the scale of the Vaud Alps, as well as on the specificity of mountain territories facing this risk. Through her professional background, she has strong skills in hydro-climatic modeling, programming, statistics, data management, as well as in cartography and land use planning. She is also passionate about mountains.

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