Vulnerabilities Alps

Observed hydrologic regime changes of Alpine rivers

Global warming affects precipitation volumes in the Alps, the contribution of rain and snow to these volumes, and the timing of snowmelt. An overall decrease in snow cover is observed during the 20th century for low- and mid-elevations. Trends are less significant at higher elevations, and snowpack even increased due to higher precipitation totals. These changes affect stream flow of mountain catchments. These changes were investigated for a large number of catchments over the period 1961–2005 in six Alpine countries (Austria, France, Germany, Italy, Slovenia and Switzerland), with catchment size varying mainly between 100 and 1000km2 (23).

Over the last decades, hydrologic regime of Alpine rivers has changed. These changes vary with the character of hydro-climatic regimes. During 1961–2005, all regimes show a consistent shift toward an earlier start of 
snowmelt flow, with a trend magnitude of 11 days, along with an increase of 18 days in the duration of the snowmelt season. Also, distinct differences have been observed between glacial- and snowmelt-dominated regimes, and mixed snowmelt–rainfall regimes (23):

• Pure glacial- and snowmelt-dominated regimes are found in the heart of the Alps. These hydrologic regimes are mainly controlled by the storage of precipitation as snow and ice during the cold months. Lowest flows occur between December and February, highest flows during spring and summer. For these regimes, winter droughts have become less severe: drought durations have decreased by an average of 25 days over 1961–2005 and volume deficits have decreased by an average of 47%. Glacial regimes, in particular, show a consistent behavior with a melting season shifted by a week earlier, an increase of 29% in the snowmelt volume, and an enhanced contribution of the glacier to the total stream flow of corresponding catchments.
• Mixed snowmelt–rainfall regimes are found in pre-alpine regions. They exhibit two low flow seasons: during the winter when part of the precipitation is stored as snowpack, and during the summer due to a combination of earlier snowpack shortage, lack of precipitation and high evapotranspiration. For these regimes, high flows are mainly driven by snowmelt during the spring and by abundant precipitation in autumn. For these regimes, winter droughts seem to have become more severe: volume deficit in the Southeastern Alps (mostly Slovenia) has increased by 10%.

Whether these trends are linked to climate change or to climate decadal variability remains an open question (23).

An analysis of runoff from 48 catchments covering different hydrological regime types in Switzerland since the 1960s found that annual runoff increased due to positive trends in winter, spring, and autumn, but found no clear trend in summer (26).  

Projections hydrologic regime changes of Alpine rivers

Global warming will have much more impact on droughts than on floods in the Alps. Simulated changes in flood magnitude are negligible whereas droughts will become more intense and last longer (31).

Climate change will have a much stronger effect on droughts than on floods in the Alps, scientists concluded in a recent study. They simulated discharge characteristics for 925 catchments in the Alps under future global warming levels of 1°C, 2°C, and 3°C. According to their results, river floods in the Alps will not change significantly in magnitude and will not last longer. The only change in floods they observed in their results is a change in seasonality: the timing of floods is expected to shift toward earlier in the year with increasing temperatures. Future droughts, on the other hand, are projected to become more intense, develop larger deficits, last longer, and become slightly more widespread with increasing temperatures (31).

These future projections of changes in floods and droughts align well with observations in the past. Observations so far do not show clear changes in flood magnitude (32) but do show earlier floods occurrences over the last decades (33) because of an earlier start of spring snowmelt season. Droughts have become more intense (34) because of decreased snowmelt and precipitation and increased evapotranspiration (35). In addition, these findings agree with the projections of other studies that show clear increases in streamflow drought deficits and intensities for the future for Central Europe and the Alps (36).

Vulnerabilities Switzerland

In comparison to other regions of the world, Switzerland is in a favourable situation, having about 5,560 m3 of water available per inhabitant per year (compared with 1,305 m3 in Germany and 690 m3 in the Netherlands) (8). In Switzerland, 83% of the drinking and industrial water demand is covered by groundwater (14).

Melting glaciers, less snow melt, more rain

The storage or retention of water in snow and glaciers is a key component of the hydrological cycle. Changes to these stores can fundamentally impact upon the availability of water, both seasonally and in the longer-term (9). For example, in snow-dominated regions, such as the Alps, Scandinavia and the Baltic, a predicted fall in winter retention as snow, earlier snowmelt and reduced summer precipitation are expected to reduce river flows in summer (10,28), when demand for water is typically at its highest.

The Alps, often described as the water tower of Europe and the source to three principle rivers (the Rhine, Rhône and Po), currently provide 40% of Europe's freshwater. The Alpine region, however, has experienced temperature increases of 1.48°C in the last hundred years —twice the global average. Glaciers are melting, the snowline is rising and the mountain range is gradually changing the way it collects and stores water in winter and distributes it in the summer months (9). While approximately 40% of the water flow was fed by snow melting between 1980 and 2009, this percentage will decrease to almost 25% by 2085 (15).

Future snowpack and summer low flows in Alpine catchments

The impact of changes in the amount of snowpack that accumulates in the winter in the Alps on summer low flow was modelled for a large number of catchments in the Swiss Alps. This was done for three future periods (2020 - 2049, 2045 - 2074, and 2070 - 2099) compared with the reference period 1980 - 2009. A moderate scenario of climate change (the so-called A1B emission scenario) was used (28).

Annual snowpack accumulation is expressed as the volume of water it represents, the so-called Snow Water Equivalent (SWE). This volume will decrease in the future because precipitation changes: more rainfall, less snow. This change has the strongest impact at lower altitudes in the mountains. The results of the model study indicate that relative decrease of annual maximum SWE in the Swiss Alps is largest at elevations below 2,200 m (a.s.l.): a reduction of 60-75% in 2070 - 2099 compared with the reference period. Above 2,200 m, this decrease is ‘only’ 20-60%. In addition to this reduction of snowpack accumulation, the snowmelt will start earlier due to the increase in air temperature: snowmelt season is projected to start up to 4 weeks earlier and last 5-20 days shorter (28).

These large changes in seasonal snow storages will greatly influence the water distribution both in time and space, especially in mountain regions with snowmelt-dominated runoff. In short: snow will play a minor role in the future for low flows, especially in July and August and for elevations around 1,500-2,000. For elevations below 1,500 m, snow storage doesn’t contribute that much to summer low flow today, and this will not change in the future (28).

It is especially snowpack accumulation decrease that will affect future summer low flows. The impacts of other changes, such as earlier snowmelt and changes in evapotranspiration, are less important. In the course of this century, runoff originating from snowmelt will contribute much less to river flow in June to August: the study shows that this contribution will decrease by more than 50% in the summer by the end of this century at the highest elevations in the Alps. At the lowest elevations, the contribution of snowmelt to summer river flow will almost disappear (28). Winter low flows in mountain catchments, on the other hand, are expected to increase (29).

The decrease in snowmelt water volume would affect reservoir management and could cause decreased water availability during the warm period for uses such as hydropower, irrigation and recreation. The decrease in low flow in spring and summer caused by the decrease in snow storages would also affect ecology of river systems. Additionally, summer low flows might become less predictable in snow-dominated catchments in the future because snow, as relatively easy predictable initial condition, will become less important and, thus, low flow predictions will rely more on the less predictable precipitation (28).

Changing river discharge regime

By 2100, Rhone runoff, for instance, is projected to change in seasonality and amount compared to the current climate such that there is a first peak of discharge that takes place 2–3 months earlier in the year, and according to the level of warming, a second peak of discharge takes place during the summer. This summer peak would occur if the number and volume of remaining glaciers were to be sufficient to contribute to such levels of discharge. Future maximum flows are reduced compared to the reference period because of the smaller contributions of a dwindling winter snow-pack. The reduction of summer discharge compared to 1961–1990 ranges from 50% to 75% (16).

The suggested increase of winter runoff and sharp decline of summer runoff in river basins such as the Rhine and the Rhone implies an increased flood risk in winter and a higher probability of drought at the end of the summer (8,10,12,15,25). Dry river beds are a rare occurrence in the northern Alps because glacier meltwaters ensure a minimum discharge in rivers during warm and dry periods of the summer, even when snowmelt no longer contributes to runoff. In the future, however, with glaciers rapidly retreating and 50–90% of the current ice mass probably disappearing (11), glacial meltwater will no longer substantially contribute to runoff.

In the European Alps, forests respond to the drought and warmer growing temperatures by increasing evapotranspiration despite depleted soil moisture. This increase leads to a decrease of water flowing into rivers and stream. During the 2003 heatwave, evapotranspiration in large areas over the Alps was above average despite low precipitation, amplifying the runoff deficit by 32% at high altitudes (30).

If the warming rate is constant, and if, as expected, glacier ice melting per unit area increases and total ice-covered area decreases, the total annual yield passes through a broad maximum: “peak meltwater”. Peak-meltwater dates have been projected between 2010 and 2040 for the European Alps (20), and for the second half of this century for the glaciers in Norway and Iceland (21).

Because of the large inter-annual variability of runoff, as a result of the sharply curtailed glacier mass in the mountains and possibly long and dry summers, the volume of summertime glacier melt waters may no longer be sufficient to feed water into river catchments at a time of the year when precipitation amounts are low and the snow-pack has already melted earlier in the year. Consequently, in some years, the Rhone may dry up partially or completely towards the end of the summer and into the early fall (16).

Future discharge characteristics may have implications for the management of water resources, e.g. for hydropower in the Alps. Furthermore, water supplies to industry, agriculture and households will be affected in the lowland areas of Western and Central Europe through which rivers originating in the Alps flow (10,16).

It has been stressed, however, that studies based on small mountainous glacierized basins overestimate the impact of climate change on downstream water flow: a study for the upper Rhone catchment (based on two regional climate models the A1B emission scenario) showed that the available water resources in the main valley and the water export from the basin (the water tower) are much less affected than small mountainous glacierized basins (22). There is an elevational dependence of climate change impacts: a severe reduction in stream flow due to the missing contribution of water from ice melt at high-elevation and a dampened effect downstream. Still, consequences for hydropower production at the upper Rhone catchment scale are possibly very significant. At the entire catchment scale a reduction of summer discharge and an increase of high flows appear to be the most significant changes to be considered by adaptation studies. However, it is unlikely that major changes in total runoff for the entire upper Rhone basin will occur in the next four decades (22).

Water quality Lake Constance affected by changing discharge regime Rhine

Lake Constance is a 536 km2 large and up to 251 m deep lake on the northern Plateau of the European Alps. The future water quality of this lake is of great importance since it is the source of drinking water for more than 4 million people and serves as well for fisheries, transportation, heat use, leisure activities, and irrigation. The Rhine River is the main tributary of this lake (24).

Sediment-laden riverine floods transport large quantities of dissolved oxygen into the receiving deep layers of lakes (undertows). Hence, the water quality of deep lakes is strongly influenced by the frequency of riverine floods. This frequency may change when climate changes (24).

Glaciers in the catchment area of the Rhine are shrinking and their contribution to Rhine discharge regime is decreasing. The river is getting more rain-fed instead. As a result, high summer discharge is shifting to spring and decreasing in height (less glacial melt water) while low winter discharge is increasing (more rain in warmer winters). According to model projections this will lead to a reduction of the number of deep density-driven underflows by 10% and a reduction of water renewal in the Lake’s deepest layers by nearly 27% at the end of this century. This way, climate change may deteriorate the lake’s water quality through a change in the water cycle of the river’s catchment that feeds the lake (24).

Europe: five lake categories

There are almost one and a half million lakes in Europe, if small water bodies with an area down to 0.001 km2 are included. The total area of lakes is over 200,000 km2; in addition the manmade reservoirs cover almost 100,000 km2. The response of European lakes to climate change can be discussed by dividing the lakes into five categories (13):

Deep, temperate lakes

Typical representatives of this class are e.g. Lakes Maggiore, Ohrid, Geneva and Constance with mean depths of 177, 164, 153 and 90 meters, respectively. Due to the great depth and relatively mild winters, there is usually no ice cover. The future climate change in Europe may suppress the turnover in deep lakes. This implies the enhancement of anoxic bottom conditions and an increased risk of eutrophication. The oxygen conditions can also be anticipated to deteriorate due to increased bacterial activity in deep waters and surficial bottom sediment.

Deep subalpine lakes in Europe, like Lake Maggiore, are experiencing a decrease in the frequency of winter full turnover and an intensification of stability. As a result, hypolimnetic oxygen concentrations are decreasing and nutrients are accumulating in bottom water, with effects on the whole ecosystem functioning (58).  A reversion in the increasing thermal stability would be possible only if global GHG emissions started to be reduced by about 2020, allowing an equilibrium mixing regime to be restored by the end of the twenty-first century. Otherwise, persistent lack of complete mixing, severe water warming and extensive effects on water quality are to be expected for the centuries to come (27). 

Shallow, temperate lakes

Balaton (600 km2, 3 m) in Hungary and Müritz (114 km2, 8 m) in Germany belong to this class. Increasing water temperatures may result in intensified primary production and bacterial composition. The probability of harmful extreme events, e.g. mass production of blue-green algae, will increase. The impacts may extend to fish life; changes in species composition and reduced fish catches will be anticipated. The use of the expression 'thermal pollution' is well justified for these lakes.

Boreal lakes

Ladoga (17 670 km2, 51 m), Onega (9670 km2, 30 m) and Vänern (5670 km2, 27 m) are the largest in this class, being also the three largest lakes in Europe. This group includes about 120 lakes with an area exceeding 100 km2. Most lakes of the boreal zone mix from top to bottom during two mixing periods each year. Shortening of the ice cover period will be the most obvious consequence of climate change in these lakes. This could improve the oxygen conditions in winter and spring.

Arctic lakes

These are mainly small water bodies in northern Scandinavian mountains and in the tundra region. Arctic lakes are generally considered to be particularly sensitive to environmental changes. Melting permafrost may seriously threaten the ecosystems of arctic lakes. In some cases the whole lake may disappear as a consequence of ground thaw and enhanced evaporation.

Mountain lakes

To this class belong all high altitude lakes in central Europe and also those located in southern Scandinavia. Even if mountain lakes were connected by channels, physical and ecological constraints limit species migration between them. In a warming climate, there is no escape route; the only possibility for survival is adaptation.

Present situation in Europe

Water demand

In the EU as a whole, energy production accounts for 44% of total water abstraction, primarily serving as cooling water. 24% of abstracted water is used in agriculture, 21% for public water supply and 11% for industrial purposes (3).

These EU-wide figures for sectoral water use mask strong regional differences, however. In southern Europe, for example, agriculture accounts for more than half of total national abstraction, rising to more than 80 % in some regions, while in western Europe more than half of water abstracted goes to energy production as cooling water. In northern EU Member States, agriculture's contribution to total water use varies from almost zero in a few countries, to over 30% in others (7). Almost 100% of cooling water used in energy production is restored to a water body. In contrast, the consumption of water through crop growth and evaporation typically means that only about 30% of water abstracted for agriculture is returned (3).

Currently, just two countries, Germany and France, account for more than 40% of European water abstraction by manufacturing industry (3).

Water supply

In general, water is relatively abundant with a total freshwater resource across Europe of around 2270 km3/year. Moreover, only 13% of this resource is abstracted, suggesting that there is sufficient water available to meet demand. In many locations, however, overexploitation by a range of economic sectors poses a threat to Europe's water resources and demand often exceeds availability. As a consequence, problems of water scarcity are widely reported, with reduced river flows, lowered lake and groundwater levels and the drying up of wetlands becoming increasingly commonplace. This general reduction of the water resource also has a detrimental impact upon aquatic habitats and freshwater ecosystems. Furthermore, saline intrusion of over-pumped coastal aquifers is occurring increasingly throughout Europe, diminishing their quality and preventing subsequent use of the groundwater (3).

Virtually all abstraction for energy production and more than 75% of that abstracted for industry and agriculture comes from surface sources. For agriculture, however, groundwater's role as a source is probably underestimated due to illegal abstraction from wells. Groundwater is the predominant source (about 55%) for public water supply due to its generally higher quality than surface water. In addition, in some locations it provides a more reliable supply than surface water in the summer months (3).

Fresh water reservoirs

Currently about 7000 large dams are to be found across Europe, with a total capacity representing about 20% of the total freshwater resource (3). The number of large reservoirs is highest in Spain (ca 1200), Turkey (ca 610), Norway (ca 360) Italy (ca 570), France (ca 550), the United Kingdom (ca 500) and Sweden (ca 190). Europe's reservoirs have a total capacity of about 1400 km3or 20% of the overall available freshwater resource (6).

Three countries with relatively limited water resources, Romania, Spain and Turkey, are able to store more than 40% of their renewable resource. Another five countries, Bulgaria, Cyprus, Czech Republic, Sweden and Ukraine, have smaller but significant storage capacities (20–40%). The number and volume of reservoirs across Europe grew rapidly over the twentieth century. This rate has slowed considerably in recent years, primarily because most of the suitable river sites for damming have been used but also due to growing concerns over the environmental impacts of reservoirs (3).

Projected future situation in Europe

Water demand

Appliance ownership data is not currently readily available for the new Member States but it is believed that rates are currently relatively low and likely to rise in the future. Higher income can also result in increased use and possession of luxury household water appliances such as power showers, jacuzzis and swimming pools. Changes in lifestyle, such as longer and more frequent baths and showers, more frequent use of washing machines and the desire for a green lawn during summer, can have a marked effect on household water use. The growth in supply within southern Europe has been driven, in part, by increasing demand from tourism. In Turkey, abstraction for public water supply has increased rapidly since the early 1990s, reflecting population growth and a rise in tourism (3).

Water stress over central and southern Europe is projected to increase. In the EU, the percentage of land area under high water stress is likely to increase from 19% today to 35% by the 2070s, by when the number of additional people affected is expected to be between 16 and 44 million. Furthermore, in southern Europe and some parts of central and eastern Europe, summer water flows may be reduced by up to 80% (4).

Supply

Runoff is estimated to increase north of 47°N by approximately 5-15% by the 2020s and 9-22% by the 2070s. North of 60°N, these ranges would be considerably higher, particularly in Finland and northern Russia (1). Average annual runoff in Europe varies widely, from less than 25 mm in southeast Spain to more than 3000 mm on the west coast of Norway. Climate change is thus going to make the distribution of water resources in Europe much more uneven than it is today. And even today's distribution is highly uneven, particularly considering the distribution of population density. Almost 20% of water resources are north of 60°N, while only 2% of people live there (2).

Not only will climate change affect the spatial distribution of water resources, but also their distribution in time. In northern Europe, the flows in winter (December to February) will increase two- to three-fold, while in spring they will attenuate considerably, in summer increase slightly and in autumn almost double by the period 2071-2100 (2).

Adaptation strategies

A number of measures exist that may potentially reduce the use of publicly supplied water. These can be broadly grouped into the categories of water saving devices; greywater re-use; rainwater harvesting and the efficient use of water in gardens and parks; leakage reduction; behavioural change through raising awareness; water pricing; and metering. Since treating, pumping and heating water consumes significant amounts of energy, using less publicly supplied water also reduces energy consumption (3).

In Denmark and Estonia, for example, a steady rise in the price of water since the early 1990s has resulted in a significant decline in household water use. Metering leads to reduced water use; in England and Wales, for example, people living in metered properties use, on average, 13% less water than those in unmetered homes (5).

Reduced water supply and increased agricultural demand for irrigation water will lead to a competitive situation in Switzerland between different uses und users, such as with downstream users. In summer, water – limited temporally and spatially – will increasingly become a scarce commodity. Thus, the necessity of suitable management will increase, which will affect the priorities, rights and prices for use. Compensation and irrigation measures will require rules as well as new infrastructure (14).

Managed aquifer recharge

Comprehensive management approaches to water resources that integrate ground water and surface water may greatly reduce human vulnerability to climate extremes and change, and promote global water and food security. Conjunctive uses of ground water and surface water that use surface water for irrigation and water supply during wet periods, and ground water during drought (17), are likely to prove essential. Managed aquifer recharge wherein excess surface water, desalinated water and treated waste water are stored in depleted aquifers could also sup­plement groundwater storage for use during droughts (18,19). Indeed, the use of aquifers as natural storage reservoirs avoids many of the problems of evaporative losses and ecosystem impacts asso­ciated with large, constructed surface-water reservoirs.

References

The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for Switzerland.

1. Alcamo et al. (2007)
2. Eisenreich (2005)
3. EEA (2009)
4. EEA, JRC and WHO (2008)
5. Environment Agency (2008a), in: EEA (2009)
6. EEA (2007), in: EEA (2009)
7. IEEP (2000), in: EEA (2009)
8. Federal Office for the Environment FOEN (Ed.) (2009)
9. European Environment Agency (EEA) (2009)
10. Andréasson et al. (2004); Jasper et al. (2004); Barnett et al. (2005), all in: European Environment Agency (EEA) (2009)
11. Swiss Agency for the Environment, Forests and Landscape (SAEFL) (2005)
12. Haeberli and Beniston (1998), in: Swiss Agency for the Environment, Forests and Landscape (SAEFL) (2005)
13. Kuusisto (2004)
14. OcCC/ProClim- (2007)
15. Bernhard and Zappa (2012), in: Matasci (2012)
16. Beniston (2012)
17. Faunt (2009), in: Taylor et al. (2012)
18. Scanlon et al. (2012), in: Taylor et al. (2012)
19. Sukhija (2008), in: Taylor et al. (2012)
20. Huss (2011), in: IPCC (2014)
21. Jóhannesson et al. (2012), in: IPCC (2014)
22. Fatichi et al. (2015)
23. Bard et al. (2015)
24. Fink et al. (2016)
25. Addor et al. (2014); Rössler et al. (2014), both in: Henne et al. (2018)
26. Birsan et al. (2005), in: Henne et al. (2018)
27. Fenocchi et al. (2018)
28. Jenicek et al. (2018)
29. Laaha et al. 2016, in: Jenicek et al. (2018)
30. Mastrotheodoros et al. (2020)
31. Brunner and Gilleland (2024)
32. Bertola et al. (2020); Blöschl et al. (2019), both in: Brunner and Gilleland (2024)
33. Blöschl et al. (2017); Fang et al. (2022), both in: Brunner and Gilleland (2024)
34. Brunner et al. (2023); Scherrer et al. (2022), both in: Brunner and Gilleland (2024)
35. Duethmann and Blöschl (2018); Moraga et al. (2021), both in: Brunner and Gilleland (2024)
36. Forzieri et al. (2014); Baronetti et al. (2022), both in: Brunner and Gilleland (2024)