Climate crisis impacting drinking water quality as forests die

The climate crisis is affecting the quality of our drinking water, according to a team of scientists, as the extreme heat is causing forests to die, removing a key water filtering system.

Researchers from the Helmholtz Centre for Environmental Research (UFZ) have modelled how climate driven deforestation impacts reservoir water quality and focused on Germany’s Rappbode reservoir.

Trees filter water and bind nutrients, leading to better drinking water with less substances, like nitrogen and phosphorous compounds.

‘This makes it more difficult for algae to develop, making drinking water treatment in the waterworks more cost-effective and easier,’ explained lake researcher and co-author Dr. Karsten Rinke. ‘Nutrient management in water conservation areas is therefore very important. Over the past decades, long-term concepts with close cooperation between forest and water management have advanced the development of large areas of forest in the Rappbode reservoir catchment area.’

The Rappbode reservoir is the largest drinking water reservoir in Germany, supplying around one million people, and has seen severe droughts from 2015 to 2020, weakening the tree population.

It’s thought the catchment area has lost over 50% of its forest due to the damage caused to trees, gravely concerning reservoir and waterworks operators.

Two different catchment areas supplying the Rappbode reservoir were studied, one of which is more agricultural and the other predominantly forest.

‘We were able to demonstrate that, for an anticipated deforestation of up to 80%, the Rappbode predam will experience an 85% increase in dissolved phosphorus concentration and a more than 120% increase in nitrogen concentration within only 15 years,’ said lead author of the study, Dr. Xiangzhen Kong. ‘The Rappbode pre-dam will thus reach nearly the same nutrient levels as the Hassel pre-dam.’

The team expects this to cause a more than 80% increase in diatoms and over 200% increase in green algae in the pre-dam, highlighting the necessity for 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 and waterworks should be adapted to the impending developments with selective water removal strategies,’ says Professor Michael Rode, last author of the study. ‘And what remains important and must be further increased: extensive, granular environmental monitoring.’

Lessons from the Rappbode reservoir can be applied to other reservoir catchment areas in similar regions.

Kong said: ‘Forest dieback as an indirect consequence of climate change has a more pronounced effect on reservoir water quality than direct effects of climate change such as elevated water temperature. We were actually surprised by the extent of this effect.’

Photo by André Künzelmann/UFZ