Climate Impacts: Field of Action Water Resources, Water Management, Coastal and Marine Protection

Low water

Extreme low-water situations are the result of a development lasting several weeks to months, mainly due to low rainfall and retention in artificial or natural water reservoirs.

Due to climate change, low-water situations may occur more frequently and more intensively in the future. This is particularly true for the Moselle, the Neckar and the Mulde for the middle of the century and for almost all rivers for the end of the century. The most significant changes are projected for parts of the Rhine.

Indicator from the monitoring on the DAS: Low water

Floods

As a result of climate change, a shift in precipitation from summer to winter is to be expected. In addition, more heavy rainfall is to be expected. Due to milder winters, the share of snow in total precipitation will decrease. This means that precipitation will be stored less frequently in the form of snow, so that the probability of flooding will increase. In recent years, annual flood levels have increased at many gauges in southern and western Germany.

Extreme flood events can overwhelm existing flood protection facilities and lead to considerable ecological and economic damage. Buildings and infrastructures can be destroyed, and in extreme situations human lives are also at risk. Pollutants such as fertilisers and pesticides or heating oil can get into groundwater and surface waters and thus considerably impair the quality of drinking water.

Especially in the low mountain ranges and in eastern Germany, an increase in flood runoff is to be expected in the future. The characteristics of extreme and damaging flood events (HQ 100 and higher) are still the subject of research.

Indicator from the monitoring on the DAS: Floodwater

Water temperature and biological water quality

Water temperature is a key parameter for the ecological status of water bodies and for the risk of eutrophication. An increase in water temperature can be expected in the future.

The ecological status of a water body is determined by the composition of the respective biocoenoses. If the water temperature rises, the solubility of oxygen in the water decreases and thus the oxygen supply of the water body. At the same time, many chemical and biological processes are accelerated by increased water temperature, which can lead to further oxygen consumption. This can lead to an oxygen deficit in the water body, which can become life-threatening for living organisms. The increased water temperatures also lead to a species shift towards heat-tolerant species.

Higher water temperature leads to eutrophication with blue-green algae formation and increases the likelihood of blue-green algae blooms (cyanobacteria). Particularly at risk are nutrient-rich waters that flow slowly or where the water is still. Eutrophication has a negative impact not only on the ecological status of the water body, but also on biodiversity and usability. Some blue-green algae are toxic to humans and animals in high concentrations.

Indicators from the monitoring on the DAS: Water temperature in lakes, Spring algal bloom in lakes

Groundwater level and groundwater quality

Groundwater is a valuable resource in Germany and serves as a source for the daily water needs of over two-thirds of the population. Groundwater is fed by precipitation and is mainly formed in winter, when little water evaporates and is transpired via plants. A possible increase in precipitation totals in winter can lead to more groundwater recharge. This phenomenon is compensated for by rising temperatures, thus increased evaporation and longer vegetation periods due to climate change. During the two record heat years of 2018 and 2019, groundwater levels dropped significantly. Low groundwater levels can be problematic for water abstraction for drinking water.

In some regions, groundwater is heavily polluted by nitrate and pesticides. In addition, the increase in air and soil temperature also raises the temperature of groundwater, which has a negative impact on its quality. Increased temperatures affect groundwater quality because they lower the oxygen content and pH values of the groundwater through increased decomposition of organic matter.

Indicator from the monitoring on the DAS: Groundwater level and spring flow

Further climate impacts

Stress or failure of flood protection systems: Technical flood protection has been used for centuries. As a rule, the statistically calculated probability of recurrence of once in 100 years serves as the basis for dimensioning the measure. With climate change, it is to be expected that higher annual peak discharges will occur and that the recurrence interval of the current design flood will shorten. It may be necessary to adapt the flood protection measures.

Flash floods (failure of drainage facilities and flood protection systems): A flash flood is a sudden localised flood with high damage potential as a result of local heavy precipitation. There is a risk of extreme heavy precipitation occurring everywhere in Germany. An accumulation and intensification of heavy precipitation is to be expected in the future.

Restrictions on the functioning of sewer networks and receiving waters and sewage treatment plants: The historically developed sewer network in German cities is overloaded by local heavy rainfall events in many cities, causing damage to settlement areas and surface waters. Increased heavy rainfall events lead us to expect increased overloading of the sewer networks and wastewater treatment plants. The performance of wastewater treatment plants is more likely to be boosted by higher temperatures in the future. Discharge of WWTP effluent into surface waters during low flows may lead to more loads.

Chemical water quality: Chemical water quality is determined by land use, intensity of use and the concentration of substances introduced. Substance inputs occur from agriculture, transport, industry and mining and private households. The degree of dilution of chemical substances in water depends on the discharge of the water body. If the runoff decreases, due to increased evaporation caused by climate change-induced warming or changed precipitation, the concentration of chemical substances increases.

Lack of irrigation water: Rising temperatures and dry periods will noticeably increase the need for irrigation in agriculture in the future. The majority of irrigation water is currently taken from groundwater. In conjunction with an increased demand for irrigation water, there could be increasing competition for groundwater as a resource.

Drinking water: In Germany, around 70 percent of drinking water comes from groundwater, followed by spring water, lakes and reservoirs, artificially enriched groundwater, bank filtrate and river water. Rising temperatures could first affect the drinking water sources near the surface, both quantitatively and qualitatively. In the long term, this also endangers groundwater, a resource that is heavily used by many users. Germs in drinking water pipes are favoured by the expected warming and endanger water quality.

Production water: About 25 percent of production water in Germany is used for industrial production. Consumption is to be further reduced through the use of comprehensive, production-integrated wastewater recycling. The industrial use of production water has declined sharply in recent years. Depending on the development of production and the economy, demand could rise or fall in the future.

Low water on the Rhine

Low water situations may occur more frequently and more intensively in the future. They have great economic significance, as they restrict inland navigation and also affect the energy industry.

Source: distelAPPArath / pixabay

Floods

Flood events are also likely to occur more frequently in the course of climate change. Extreme floods can overwhelm existing flood protection facilities and cause considerable damage.

Source: analogicus / pixabay

Water quality of lakes

Higher temperatures lead to higher water temperatures in lakes. This increases the risk of eutrophication. The probability of a blue-green algae bloom also increases.

Source: Jan-Mallander / pixabay

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Ocean temperature and ice cover

Ocean temperature and the ice cover associated with it play a decisive role in the balance of marine ecosystems. The global warming trend of the oceans during the last decades could also be observed in the North and Baltic Seas. In the future, the temperature increase in the winter months will probably be higher than in the summer months. In addition, short-term marine heat waves will occur more frequently, longer and more intensively in the future. They manifest themselves in an extreme rise in surface water temperature on a regional scale and can last from weeks to months. They can have severe, sometimes irreversible impacts on marine ecosystems.

Over the last century, the number of mild winters has increased and the number of cold winters with favourable conditions for ice formation has decreased for the North Sea and Baltic Sea. This trend towards decreasing ice cover will continue. A complete absence of ice formation is not expected for the Baltic Sea and North Sea by the end of the century.

Indicator from the monitoring on the DAS: Water temperature of watercourses

Sea level rise

The German North Sea and Baltic Sea coasts are exposed to increasing risks as a result of sea-level rise. On the German North Sea coast, regional sea level rise between 1900 and 2011 was about 1.6 to 1.8 millimetres per year. On the German Baltic Sea coast, rates of change of about one millimetre per year were measured over the same period. Projections give a likely range for global mean sea level rise by mid-century (2031 to 2060) of 0.23 to 0.40 metres and by the end of the century (2071 to 2100) of 0.61 to 1.10 metres (each relative to the period 1986 to 2005). It can be assumed that the values for the German North Sea coast correspond to the global sea level rise.

Natural changes on coasts

On the German North Sea coast, tidal flats and salt marshes in the Wadden Sea are particularly at risk of potential decline due to sea-level rise. The Wadden Sea can grow along with increased sediment deposition and thus partially compensate for rising sea levels. However, if this growth in balance with rising sea levels is not sufficient, the Wadden Sea could develop from a mudflat-dominated to a lagoon-dominated system. This would require major changes in the ecosystems there. Salt marshes are characterised by plant growth in the intertidal zone and fulfil important functions as habitats for fauna and flora and as coastal protection. The consequences of climate change may lead to changes in species composition and a decreasing width of salt marshes.

In the Baltic Sea, the outer coasts and especially the islands and the Fischland-Darß-Zingst peninsula are most affected by coastal recession and land loss. Steep coasts are also subject to high risks from erosion.

Damage to or destruction of coastal settlements and infrastructure

Settlements and infrastructure in coastal regions worldwide are threatened by the consequences of climate change. The extent of damage in coastal areas caused by storm surges and floods is highly dependent on the resilience of coastal protection systems. If protective structures fail, functional impairment of infrastructure and damage to settlements can be possible consequences. Considering the sea level rise expected by the end of the century, flood events that statistically currently occur once every 100 years could occur annually in the future.

The greatest potential for damage is for residential and commercial buildings. In the infrastructure sector, port facilities and sea waterways are particularly at risk due to their function and dependence on water levels.

Overloading of drainage facilities in flood-prone areas

Low-lying and thus flood-prone coastal areas are mainly found on the German North Sea coast, but there are also flooded areas on the Baltic Sea coast in the inner coastal waters, the Bodden and the Haffen. Most lowland areas are permanently drained by a developed drainage system. They are predominantly used for agriculture, and there are also isolated settlement areas and tourist uses.

Overloading of the drainage facilities may occur more frequently in the future due to rising sea levels and low tidal water levels as well as heavier rainfall. The increasing reduction in the water level gradient between inland and external water levels reduces the time window for sieve drainage and the general drainage potential. From the middle of the century at the latest, a severe restriction of sieve capacities is to be expected. The resulting increase in the need for pumping stations to drain coastal lowland areas is likely to entail considerable investment.

Further climate impacts

Water quality and groundwater salinisation: Higher water temperatures and the increase in atmospheric CO2 concentration as well as anthropogenic nutrient inputs in coastal areas increase eutrophication and oxygen deficiency and lead to ocean acidification. Groundwater salinisation in coastal areas may be exacerbated by climate change. Climate change may also cause a decrease in salinity in the Baltic Sea.

Currents and tidal dynamics: On the German North Sea coast there is a change in tidal amplitude, which is also related to sea-level rise. For the North Sea, changes in the inflow of salty Atlantic water over the northern edge of the North Sea and over the Strait of Dover are expected. An increase in westerly wind conditions may increase the inflow from the North Sea into the Baltic Sea.

Sea state: The projection of future developments in sea state is associated with great uncertainties. A slight increase in wind events over the North Sea and Baltic Sea can be assumed in the future, especially an increase in westerly wind conditions. An increase in wave heights could occur in the German Bight and on west- and northwest-exposed coastal sections of the Baltic Sea.

Storm surges: No significant changes in the strength, duration and frequency of storm surges are expected for the middle and end of the century. Sea level rise associated with climate change will cause higher water levels during storm surges. Series of storm surges in combination with sea-level rise will lead to an additional aggravation of the drainage problems that already exist today in the low-lying low-lying areas of northern Germany that are diked in.

Increased stress or failure of coastal protection systems: Coastal protection systems are increasingly exposed to severe stresses as a result of the impacts of climate change. The main influencing factors are sea level rise, an increasingly higher baseline level for storm surges and the impairment of natural protective functions in the coastal zone.