Climate change is affecting the quality of drinking water according to a new study by scientists at UFZ.

Heat waves, drought, floods, forest fires – the consequences of climate change are increasing and are changing the environment according to scientists from the Helmholtz Centre for Environmental Research in Leipzig, Germany.

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

UFZ hydrologist, Professor Michael Rode, said: “Over the past four years, the Rappbode catchment area, characterized by conifers, primarily spruce, has lost over 50% of its forest. 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 water quality. The fewer nutrients, such as nitrogen or phosphorous compounds, contained in reservoir water, the better it is for drinking water treatment.

Climate change-induced deforestation

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

The rapid forest decline in the eastern Harz region is now a matter of grave concern for the reservoir and waterworks operators. The UFZ team has now investigated the effects of climate change-induced deforestation on reservoir water quality in their model study.

The study was based on data from the TERENO (Terrestrial Environmental Observatories) environmental observatory network, in which the UFZ is a participant with the Harz/Central German Lowland Observatory. The team also used data from the international ISIMIP project (Inter-sectoral Impact Model Intercomparison Project) to predict future climate change.

UFZ environmental scientist and lead author of the study, Dr Xiangzhen Kong, said: “We were able to access environmental data from a period of over ten years, providing us with a solid set of data. We first fed these data into a model in order to estimate the climate-related effects on the nutrient balance in the catchment area.

“The resulting data were then processed in a reservoir ecosystem model with which we were able to determine the effects of different deforestation scenarios on the predicted water quality for 2035.”

Agricultural influence

The Rappbode reservoir is supplied by three different catchment areas, two of which were included in the study. Before the water from the two catchment areas flow into the large Rappbode reservoir, it is first retained by an upstream pre-dam. The agricultural influence results in a significantly higher nutrient content in the water in the Hassel pre-dam than that in the Rappbode pre-dam.

Kong added: “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.”

“We were able to demonstrate that, for an anticipated deforestation of up to 80%, the Rappbode pre-dam will experience an 85% increase in dissolved phosphorus concentration and a more than 120% 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.”

If that happens it will result in a more than 80% increase in diatoms and more than 200% increase in green algae in the Rappbode pre-dam and highlight the coming necessity for a wide range of adaptations in drinking water management.

Rode advised: “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.

“What remains important and must be further increased: extensive, granular environmental monitoring.”

Image: The Rappbode reservoir in the Harz region is surrounded by forests and is the largest drinking water reservoir in Germany. Credit: André Künzelmann/ UFZ.