Climate change affects the entire water cycle. Surface and underground waters are equally exposed to changes in water quantity and quality. This has direct repercussions on hydropower, water supplies, urban drainage, navigation, agriculture, ecology and water-induced natural hazards.
The “Climate Change and Hydrology in Switzerland” (CCHydro) project initiated by the Federal Office for the Environment (FOEN) examined the repercussions of climate change on the Swiss water balance. For the first time, scientifically reliable information could be given on the changes in the Swiss water balance by the end of the 21st century.
In order to improve understanding of the hydrological process and update the knowledge base, the project “Climate Change and its Consequences on Hydrology in Switzerland” (Hydro-CH2018) was initiated. It represents one of the core topics of the newly formed Swiss Government network National Centre for Climate Services (NCCS).
New hydrological scenarios will be created on the basis of the updated 2018 climate scenarios. These will focus mainly on the areas of extreme events (floods and low water), natural and man-made reservoirs as well as surface water temperatures and ecology. Climate services for water will also be developed and made available. They give scientifically-based information and data on the past, present and future water balance. They will help authorities, policy makers, businesses and society to reduce climate-induced risks.
In the alpine region, the rise in temperature is the most important factor influencing the seasonal changes in discharges: The snow line is rising and winter snow reserves and glacier volumes and areas are shrinking.
The seasonal discharge distribution (regime) will change throughout almost the whole of Switzerland. Discharges will increase considerably in many regions in winter but will decrease in summer. Regimes that are largely unknown today will occur, with a seasonal maximum discharge in winter and a very low minimum in August. The overall total annual discharges in Switzerland will change relatively little in the near future (to 2035) and will even increase temporarily in glaciated regions. In the more distant future (2085), total discharges will generally decrease slightly.
By the end of this century the area of glaciation in the Swiss Alps will have been reduced even further. Glaciers will only be found in the high-elevation regions of the Bernese and Valais Alps. Depending on the model and climate scenario used, a loss of 60 to 80% of the current Swiss glacier area can be assumed.
By the end of the century the greatest volume of ice will be located in the Rhone basin (Valais), where some 80% of the Swiss glacier volume is currently found. In contrast, the Rhine basin will lose all its glaciers apart from a few stretches of ice in the Bernese Oberland. The Engadine and Ticino will be totally ice free by the end of the century.
Snow is an important water resource in Switzerland. In alpine regions snow melt is the principal component of the discharge for a third of the year (March to June). The snow cover responds very sensitively to changes in temperature and precipitation, making it a good indicator of changes in climate. Due to the increase in temperature, a higher proportion of the precipitation is already falling as rain rather than snow, particularly at lower levels. Snow melt is also occurring earlier in the season due to the higher temperatures. This is shortening the period of snow cover and reducing the quantity of water stored temporarily in the snow. This trend will further intensify by the end of the century.
A warming atmosphere can hold more water vapour, leading to a greater potential for heavy precipitation. Due to rising temperatures, more precipitation also falls as rain rather than snow (particularly in spring and autumn in alpine and prealpine catchments and also in summer in very high elevation regions). Therefore an increase in the frequency and intensity of medium and large flood events and an extension of the potential flood period in spring and late autumn can be expected. It is not known how the atmospheric circulation and therefore the frequency of weather conditions triggering floods will develop due to climate change.
At the same time, the increase in hot and dry summers, more frequent and longer lasting low water periods can be expected, especially in late summer.
Soil moisture and evaporation
The CH2018 climate projections anticipate hotter and drier summers with longer and more frequent droughts for Switzerland. More water can also evaporate at higher temperatures. All these factors will lead to a reduction in soil moisture in summer. Whether or not more water will actually be lost to evaporation in a future climate will depend on how much is still available in the soil to evaporate and what the water requirement of vegetation will be. Adaptations in agriculture and forestry (e.g. climate compatible crops, irrigation) will also be necessary in Switzerland.
Water temperature, quality and ecology
Increasing air temperatures, the introduction of warm water from building cooling and wastewater treatment systems and the lack of shade-giving bank vegetation have allowed the water temperatures of many surface waters to rise significantly over recent decades.
Higher water temperatures result in less dissolved oxygen in the water. Bioactivity also increases causing oxygen demand to increase. In addition, diseases which are a health hazard for aquatic organisms spread more widely in warmer water. Lower water levels in the summer months will cause further increases in water temperature and endanger the survival of some organisms. Furthermore, if discharges are low, wastewater is less well diluted and water quality suffers.
The aquifers of the Jura, the Central Plateau and the Prealps are recharged by rain and snow melt. It can be assumed that they will fill in winter and will empty in spring due to the snow melt deficit. Groundwater recharge in alpine river valley gravels will also reduce in line with the decreased snow and glacier melt in summer. During the more frequent and longer periods of drought forecast by the climate scenarios, groundwater exfiltration into the rivers is likely to increase. Changes in groundwater recharge will affect groundwater temperature as well as its quantity. A rise in groundwater temperature will therefore be observed in the aquifers fed by river water infiltration, and also in urban areas, e.g. due to heat input from building cooling systems. Rising groundwater temperatures may lead to an increase in microbial activity and a fall in the oxygen concentration and in extreme cases iron and manganese precipitation.
An increase in heavy precipitation, thawing of the permafrost and melting of glaciers may lead to more erosion, landslides and rockfalls. More material will then be available in the water for sediment transport. Higher sediment transport changes the discharge characteristics and channel morphology. This can have negative consequences for flood protection, restrict the output of hydropower plants and harm the water ecology.
Last modification 08.11.2018