Climate change is affecting drinking water quality

Heat waves, drought, floods, forest fires – climate change's consequences are increasing and changing our environment. New research indicates that it has impacted the drinking water quality.

Heat waves, drought, floods, forest fires – climate change’s consequences are increasing and changing our environment. New research indicates that it has impacted the drinking water quality.

Nanjing Institute of Geography and Limnology, Helmholtz Centre for Environmental Research (UFZ), Technische Universität Braunschweig and the University of Potsdam has found that the drinking water quality in reservoirs has deteriorated due to deforestation.

A prime example is the countryside in the catchment area for the Rappbode reservoir in the eastern Harz region. This is Germany’s largest drinking water reservoir and provides drinking water for roughly one million people. Long periods of drought over the years from 2015 to 2020 have severely weakened the Harz region’s tree population that parasites such as bark beetles have been able to propagate. This further exacerbated the effect: The trees were further damaged and quickly died off.

Drinking water quality impacted by deforestation

“The Rappbode catchment area, characterised primarily by spruce, has lost over 50 per cent of its forest,” said UFZ hydrologist and last author Prof. Michael Rode. “This massive forest dieback is advancing rapidly and is dramatic. This will have consequences for the drinking water reservoir.”

Forests play a key role in the water cycle. They filter the water and bind nutrients and are therefore necessary for good drinking water quality. The fewer nutrients – i.e. nitrogen or phosphorous compounds – contained in reservoir water, the better it is for drinking water treatment.

“This makes it more difficult for algae to develop. It makes drinking water treatment in the waterworks more cost-effective and easier,” explains UFZ lake researcher and co-author Dr Karsten Rinke. “Nutrient management in water conservation areas is therefore very important. Long-term cooperation between forest and water management has advanced the development of large forest areas in the Rappbode reservoir catchment area.” The rapid decline in the eastern Harz region is now a matter of grave concern.

Drinking water quality investigated

The team investigated the effects of climate-induced deforestation on reservoir water quality in their model study. This study was based on data from the TERENO (Terrestrial Environmental Observatories) environmental observatory network. The UFZ collaborated with the Harz/Central German Lowland Observatory. “We were able to access environmental data from over ten years. That provided us with a solid set of data,” said Dr Xiangzhen Kong, a UFZ environmental scientist and lead author of the study.

The team used data from the international ISIMIP project (Inter-sectoral Impact Model Intercomparison Project) to predict future climate changes. “We first fed these data into a model to estimate climate-related effects on nutrient balance,” explains Kong. “The resulting data were then processed in a reservoir ecosystem model. We could determine the effects of different deforestation scenarios on the predicted water quality for 2035.

The Rappbode reservoir is supplied by three different catchment areas, two of which were included in the study. “The Hassel catchment area is characterised by agriculture, while that of the Rappbode is predominantly forest – at least that was the case before the spruce forests died,” said Kong. Before the water from the two catchment areas flows into the large Rappbode reservoir, it is first retained by an upstream pre-dam. The agricultural influence results in significantly higher nutrient content in the Hassel pre-dam water than in the Rappbode pre-dam.

Conclusions about drinking water quality

“We were able to demonstrate that, for anticipated deforestation of up to 80 per cent, the Rappbode pre-dam will experience an 85 per cent increase in dissolved phosphorus concentration and a more than 120 per cent increase in nitrogen concentration within only 15 years. The Rappbode pre-dam will thus reach nearly the same nutrient levels as the Hassel pre-dam,” said Kong. This will result in a more than 80 per cent increase in diatoms and more than 200 per cent in green algae in the Rappbode pre-dam. These results highlight the necessity for a wide range of adaptations in drinking water management.

“Nutrient input to reservoir catchment areas should be reduced even more than previously. Reforestation projects with drought-resistant tree species should be further promoted. Waterworks should be adapted to the impending developments with selective water removal strategies,” said Rode. “What remains important must be further increased: extensive, granular environmental monitoring.”

The results for the Rappbode reservoir can be applied to other reservoir catchment areas in similar regions. “Forest dieback as an indirect consequence of climate change has a more pronounced effect on reservoir water quality than direct effects such as elevated water temperature. We were surprised by the extent of this effect”, said Kong.

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