Overall, Germany is a country rich in water. Nevertheless, some regions may already experience seasonal difficulties with regard to sufficient water availability.
As a result of climate change, longer and more frequent periods of drought and low water in the summer months are to be expected. In addition, rising temperatures lead to an overall increase in evaporation, allowing less water to seep and infiltrate the groundwater. This affects groundwater recharge. Compared to the long-term average, months with below-average low groundwater levels are becoming significantly more prevalent. Especially precipitation deficits occurring over several years in a row lead to sinking groundwater levels.
Both the change in temperature and the change in precipitation influence the surface runoff, which has an impact on groundwater:
In the winter months, precipitation increases, but then it hits already water-saturated or frozen soil and cannot seep away.
In the summer months, soils dehydrate more severely due to higher temperatures and reduced precipitation.
If precipitation occur more frequently in the future in the form of heavy rainfall, they cannot or can only hardly be absorbed by dry soils and therefore flow off to a large extent above ground.
The trend towards increased groundwater low-levels is particularly distinctive in the low-precipitation areas of north-eastern Germany, i.e. in areas with annual precipitation of less than 700 mm.
In some regions, on the other hand, rising groundwater levels may occur due to increasing winter precipitation:
Waterlogging can lead to the saturation of arable land (rotting of seed, postponement of the cultivation of fields), forest areas or biotopes. Additionally, nitrate can be washed out of arable land.
A change in the possible uses of gardens as well as agricultural and forestry land can be a further consequence.
Flooding of lower lying areas, for example streets or underpasses, is possible as well.
Both rising and sinking groundwater levels can lead to damage to buildings, for example in the basement or on the foundations, and through subsidence with damage to the masonry.
However, falling groundwater levels can also have negative effects on groundwater-dependent terrestrial ecosystems such as alluvial zones, wetlands or moors - with consequences for nature conservation and climate protection.
It is true that both commercial and private water use in Germany has declined significantly over the past 25 years. In 2004, it fell below the critical water use index of 20 % for the first time, that is to say no more than 20 % of the potential water supply is used. However, there are significant regional differences, and climate change poses new challenges for water utilities, for example to cover seasonal peak consumption.
In addition, the drinking-water suppliers have recently recorded an increase in demand again. While consumption in the previous ten years had been constant at around 122 litres per capita and day, it rose to 127 and 125 litres respectively in the heat years 2018 and 2019.
Reduced water availability leads to restrictions in energy, agriculture and forestry sectors and has a negative impact on ecosystems and their biodiversity.
The inland navigation is also affected. As early as 2015, low and sometimes even extremely low water levels and flows over long periods of time have been common in the federal waterways.
The industry is also affected by low water levels, for example when cooling water cannot be taken from rivers. In addition, high water temperatures in the rivers can limit the discharge of hot water from the cooling water circuits of power plants into the rivers, so that production can be reduced.
Rising temperatures and stronger insolation heat up the upper layers of surface waters. The lower flow velocities at low tide also cause the water to heat up faster. This has an impact on the water quality and influences the biology in the water bodies as well. This is because warmer water has a lower oxygen content, which affects animals and plants. Various types of fish and invertebrates are subject to great heat stress.
At higher temperatures there is a stronger plant growth especially of algae, which results in a reduced oxygen concentration especially in river lakes.
The toxins formed by the algae can lead to symptoms of poisoning in humans and animals if ingested in large quantities.
If the runoff quantities are reduced, discharges into water bodies are also less diluted, which leads to higher nutrient or pollutant concentrations. In the summer of 2018, for example, the concentrations of calcium, potassium, sodium and chloride as well as the electrical conductivity reached peak values at individual measuring points on the federal waterways that had not been measured there for 20-25 years. Nickel and arsenic were detected in slightly elevated concentrations at several measuring points.
Oxygen deficiency in combination with accumulating nutrients can disturb the ecological balance of the water body (eutrophication). Especially smaller standing waters are affected. But also in the North Sea and Baltic Sea an increase of oxygen deficiency and blue-green algae bloom can be observed with rising temperatures. This is further reinforced by the over-fertilization of the oceans.
The trend towards rising temperatures affects various bodies of water and types of lake, from the North German lowlands (Großer Müggelsee), through lakes in the Alps and the Alpine foothills (Lake Constance) to the low mountain ranges (Saidenbach dam). At Lake Constance, for example, the temperature increase in the season (March and October) between 1971 and 2017 totalled about two degrees, and in the Saidenbach dam between 1977-2016 as much as three degrees celsius.
Rising sea temperatures have far-reaching effects on the entire marine ecosystem. Species adapt their distribution areas, die out (locally or regionally) and other species occupy these ecological niches. Indirect side effects of climate change, such as oxygen deficiency and ocean acidification, are also contributing to the fact that the entire marine food web is changing with the diversity, composition and distribution of species.
Floods, which may occur more frequently in the future, can also affect the quality of water bodies and groundwater.
As a result of climate change, a shift in precipitation from summer to winter is expected. In addition, increased heavy rainfall is to be expected in winter. Due to milder winters, the amount of snow in total precipitation will decrease. This means that precipitation is less often stored in the form of snow, which increases the probability of flooding.
Urban centres of population are particularly at risk, as sealed surfaces prevent water from seeping into the subsoil. Even now, short but intense heavy rainfall often leads to drainage problems in urban areas.
Floods can cause pollutants such as fertilizers and pesticides to enter groundwater as well as surface water and impair water quality. The flooding of industrial and sewage plants as well as private fuel oil tanks can also cause pollutants to enter the water. This can lead to considerable impairment of the drinking water quality and thus to health problems.
In addition, the damage or destruction of buildings and infrastructure can cause great and long-term economic damage.
In some regions of Germany, heavy rainfall events can also increase the risk of flash floods.
Coastal and marine protection
The melting of large quantities of continental ice and the expansion of warming seawater are causing sea levels to rise. According to a special report by the Intergovernmental Panel on Climate Change (IPCC) in 2019, the mean global sea level rose by 16 centimetres in the 20th century and could rise by another 61 to 110 centimetres by 2100. The increase is currently 3.4 millimetres per year and is accelerating.
The rates of seawater level rise in the North Sea and Baltic Sea are slightly below these global rise rates and, excluding the influence of land subsidence, amount to 1.6 to 2.9 millimetres per year in some German coastal areas.
As sea levels rise, the starting level for storm surges increases. High storm surge water levels could be reached faster and last longer in the future. By the middle of the 21st century, floods, which currently occur once in 350 years, could occur once in 100 years. On some coastal areas, such high-water levels could occur even more frequently. Heavy storm surges hit the German Baltic coast at the beginning of 2017 and 2019. In Wismar a water level of 1.83 m above mean sea level was reached in the evening of January 3, 2017, which was exceeded on January 2, 2019 at the same location with 1.91 m.
In addition to sea-level rise, storm surge water levels are rising, especially in the estuaries of large rivers, due to increasing embankment construction and the blocking of tributaries, e.g. the Ems, Weser and Elbe. This has considerably reduced the natural flood areas.
The waves that reach the coasts are influenced in their height and power not only by the water level but also by wind and swell. In the North Sea and Baltic Sea, especially in the winter months, higher storms are to be expected. By 2030, storm surges at the North Sea will be about 10 to 30 cm higher than today. The figures from the Vulnerability Report show that by 2100 an increase of even 30 to 110 cm can be expected. This is a challenge for coastal protection, which according to current estimates must be significantly expanded by the end of the 21st century.
In addition to damage to buildings and infrastructure, the consequences of storm surges are the penetration of salt water into the interior of the country. The salt water thus reaches the fresh water of ground and surface waters.
Also the rising sea level leads to a shift in the mixing zone of fresh and salt water. This restricts the regional water availability and quality and changes the soil and thus the conditions for forestry and agriculture. Salinization of fresh water can also be a problem for industry and commerce if production depends on high water quality.