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 (22).

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 (22):

• 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 (22). 

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 Austria

River discharge: Upper Danube at Vienna

The Danube River is the second largest and second longest European river, crossing 10 countries and draining areas that belong to 19 countries in the historically turbulent and culturally diverse Central and Southeast European regions. A large number of regional climate model (RCM) simulations driven by an intermediate (RCP4.5) and high-end (RCP8.5) scenario of climate change shows that winter discharge increases for the Upper Danube at Vienna while the seasonal discharge peak in spring occurs earlier and the summer discharge decreases (27). This is due to increasing winter and spring, and decreasing summer precipitation, earlier snow melt, and higher evaporation in the summer.

These effects are more pronounced in a more distant future (2071-2100) than in an earlier period (2021-2050). These findings confirm projected patterns of changes in discharge seasonality found in previous studies, both on already observed trends (28) and on future projections (29). The expected decrease in future mean annual discharge of the Danube at Vienna is relatively small: -3.3% in the median of all analysed scenarios for the period of 2071-2100 (27).

This typical change pattern of increasing discharge in winter and decreasing discharge in summer is also apparent in projections for smaller tributary catchments of the Upper Danube. For the high-end scenario (RCP8.5) winter discharge is markedly higher and summer discharge marked lower than for the intermediate scenario (RCP4.5), both for the Upper Danube and its tributaries. This is due to lower summer precipitation and higher summer temperature (and hence evaporation) (27).

River discharge: general pattern

In most Austrian climate change scenarios for hydrologic modelling, the summer rainfall is slightly decreased and winter rainfall increased, while the annual amount of rainfall remains rather stable except for the dry and flat basins in southern and eastern Austria, where the annual amount of rainfall is decreased. In general, the daily variability of rainfall is slightly increased (1).

Preliminary studies (2) indicate that in the Alpine basins the seasonal runoff pattern will change. Low-flow conditions occurring now in early winter will appear during fall because of increased temperatures. The melting period will also start earlier; the occurrence of monthly runoff maxima is basin dependent and will fall into the time period March to June. The number of days with snow cover will decrease as will the frequency and duration of frost periods. The increase in temperature and thus in evaporation is higher than the changes in rainfall and, therefore, there is a tendency towards a decreased runoff, which is only counterbalanced by higher runoff during winter.

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).

The frequency of low-flow conditions, especially in late summer and fall, increases. Because evaporation increases and soil moisture decreases as will the groundwater recharge, flat areas will experience hydrological conditions that are more distinct and severe than those in the mountains (1).

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 heat wave, evapotranspiration in large areas over the Alps was above average despite low precipitation, amplifying the runoff deficit by 32% at high altitudes (30).

Analysis of satellite data from the 1980s and early 1990s shows that lowlands around the Alps experience about 3–4 weeks less snow cover than they did historically (3). This tendency can be expected to accelerate in a warmer climate with the consequence that early seasonal runoff will increase and thus lead to drier soil and vegetation in summer.

Southeastern Alpine forelands

The southeastern Alpine forelands, a transition zone between the Alpine and the Mediterranean region, is especially vulnerable to climate change. The temperature in this Alpine foreland region shows a strong summer temperature trend with an increase of near 0.7 °C per decade from 1971 to 2016, much stronger than the recent global warming trend of near 0.2 °C per decade since the 1970s (24) and the trend of near 0.5 °C per decade over the European Alpine region (25). The precipitation trend is not distinct; the summer precipitation only shows a slight decrease near 1% per decade from 1971 to 2016. Yet, climate projections suggest that summer precipitation decreases in the order of 10-20% within the twenty-first century are realistic (25). 

In this region, streams show a decreasing trend of low-flow runoffs (26), which means the catchments become drier. For one of these catchments, the Raab catchment in southeastern Styria, low flow in the summer months was studied (23). Summer streamflow of the river Raab is now decreasing by approximately 3% per decade. The study shows that climate change projections for this region at the end of this century may lead to a summertime runoff decrease up to more than 40% (moderate climate change) or more than 70% (strong climate change). Changes in land use may decrease, and changes in water management (irrigation) may decrease the impact of climate change.

Lake Neusiedl

A particular sensitive hydrological system to climate change is the lake Neusiedl at the Austrian/Hungarian border. This shallow lake (~ 1,5 m) has no natural drain and its water level is mainly defined by precipitation on the lake and evaporation (4,18). Since the formation of the lake about 13,000 years ago it has dried up more than 10 times. The lake totally disappeared from 1740–1742, 1811–1813, and most recently from 1866–1871 (19). Till the beginning of the 20th century, when the Main Regulation Channel (Einserkanal) was built, Lake Neusiedl had no outflow. Since 1965 the sluice gate management via this channel is regulated aimed to prevent flooding (18).

The observed increase of temperature and also sunshine duration within the last decades enhanced the lake evaporation by 10 % (5) for the period 1991–2004 compared to 1961-1990. Within the last 15 years also a weak decrease of precipitation has been observed (~ 6 %) and the lake level was sinking, leading to some troubles in tourism (sailing).

Assuming a temperature increase of 1.8°C within the next 35 years a further increase of the lake evaporation by 15 % was found. The return period for reaching critical lake levels for sailing has changed from ~ 30 years in 1961-1990 to 12 years in 1991-2004 and in the scenario for 2040 the critical level was reached nearly every third year, assuming no change in yearly precipitation (5).

Lower water levels might lead to a reduction of touristic and sailing attractiveness, and an increase of nutrient concentrations and enhanced growth of phytoplankton and cyanobacteria (18). Lake Neusiedl is a famous European bird breeding region and a vanishing of this lake would have important impact on the European fauna. As summer tourism is an important regional economic factor, the possible impact of climate change on lake Neusiedl has a high priority for regional authorities (1).

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.

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 (8).

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 (12). 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 (8).

Currently, just two countries, Germany and France, account for more than 40% of European water abstraction by manufacturing industry (8).

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 (8).

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 (8).

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 (8). 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 (11).

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 (8).

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 (8).

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% (9).

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 (6). 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 (7).

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 (7).

Adaptation strategies

EU policy orientations for future action

According to the EU, policy orientations for the way forward are (14):

• Putting the right price tag on water;
• Allocating water and water-related funding more efficiently: Improving land-use planning, and Financing water efficiency;
• Improving drought risk management: Developing drought risk management plans, Developing an observatory and an early warning system on droughts, and Further optimising the use of the EU Solidarity Fund and European Mechanism for Civil Protection;
• Considering additional water supply infrastructures;
• Fostering water efficient technologies and practices;
• Fostering the emergence of a water-saving culture in Europe;
• Improve knowledge and data collection: A water scarcity and drought information system throughout Europe, and Research and technological development opportunities.

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 (15), 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 (16,17). 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.

Measures

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 (8).

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 (10).

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 Austria.

1. Federal Ministry of Agriculture, Forestry, Environment and Water Management (2010)
2. Holzmann et al. (2008), in: Federal Ministry of Agriculture, Forestry, Environment and Water Management (2010)
3. Baumgartner and Apfl (1994), in: Federal Ministry of Agriculture, Forestry, Environment and Water Management (2010)
4. Boroviczeny, F. et al. (1992), in: Federal Ministry of Agriculture, Forestry, Environment and Water Management (2010)
5. Eitzinger et al. (2005), in: Federal Ministry of Agriculture, Forestry, Environment and Water Management (2010)
6. Alcamo et al. (2007)
7. Eisenreich (2005)
8. EEA (2009)
9. EEA, JRC and WHO (2008)
10. Environment Agency (2008a), in: EEA (2009)
11. EEA (2007), in: EEA (2009)
12. IEEP (2000), in: EEA (2009)
13. Kuusisto (2004)
14. Commission of the European Communities (2007)
15. Faunt (2009), in: Taylor et al. (2012)
16. Scanlon et al. (2012), in: Taylor et al. (2012)
17. Sukhija (2008), in: Taylor et al. (2012)
18. Soja et al. (2013)
19. Österreichisch – Ungarische Gewässerkommission (1996), in: Soja et al. (2013)
20. Huss (2011), in: IPCC (2014)
21. Jóhannesson et al. (2012), in: IPCC (2014)
22. Bard et al. (2015)
23. Hohmann et al. (2018)
24. Willett et al. (2016); GISTEMP Team (2017), all in: Hohmann et al. (2018)
25. Gobiet et al. (2014), in: Hohmann et al. (2018)
26. Laaha et al. (2016), in: Hohmann et al. (2018)
27. Stanzel and Kling (2018)
28. Fürst et al. (2007); Pekarova et al. (2008), both in: Stanzel and Kling (2018)
29. Stanzel and Nachtnebel (2010); Koch et al. (2011); Prasch et al. (2011); Kling et al. (2012); Stagl and Hattermann (2015); Wagner et al. (2017), all in: Stanzel and Kling (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)