Droughts, forest fires and pests are gnawing at the forests in many places: researchers have now shown the challenge that climate change-related losses can pose for the provision of drinking water, using the example of the Rappbodetalsperre in the Harz Mountains. The damage to the forests in the catchment area of this largest drinking water dam in Germany is a serious threat to the water quality. The scientists say that the indirect consequences of climate change are often still greatly underestimated.
It is well known that forests play an important role in the water cycle: in addition to their storage function, they filter water, bind pollutants or problematic fertilizers. They therefore make a significant contribution to improving the water quality. Forests are therefore of great importance, especially in catchment areas of water bodies for drinking water supply, such as dams. However, climate change is threatening forest stands and their productive capacity in many places: heat waves, droughts, floods and fires affect the trees and weaken their resistance to other threats such as bark beetle infestation.
The catchment area of the Rappbodetalsperre in the eastern Harz Mountains, which supplies around one million people with drinking water, is also affected by these harmful effects. The scientists around Michael Rode from the Helmholtz Center for Environmental Research (UFZ) in Magdeburg chose this system as a model to investigate the effects of climate-related deforestation processes on the water quality of dams. "The catchment area of the Rappbode, which is characterized by coniferous forest - mainly spruce - has lost over 50 percent of its forest in the past four years," says Rode. “This massive loss is rapid and dramatic. It seems clear that this will not be without consequences for the drinking water reservoir.”
Indirect consequences of climate change
As the team explains, the influence on the nutrient content is particularly important: there should be little nitrogen and phosphorus in the water. "Then fewer algae can develop, and the drinking water treatment in the waterworks runs more cost-effectively and with less effort," says Karsten Rinke from the UFZ. “Nutrient management in water protection areas is therefore very important. Long-term concepts, in which forestry and water management work closely together, have promoted the development of large forest areas in the catchment area of the Rappbodetalsperre over the past few decades.
In order to be able to more accurately assess the threats to the water quality, the team evaluated comprehensive information and carried out model simulations. "We were able to access environmental data from a period of more than ten years and thus had a solid data basis," says lead author Xiangzhen Kong from the Chinese Academy of Sciences in Nanjing. To forecast future conditions, the team used information from an international climate research project. "We first fed this data into a model in order to be able to model the climate-related effects on the nutrient balance of the catchment area," says Kong. "The data generated from this was then run through a reservoir ecosystem model, with which we were able to determine the effects of different deforestation scenarios on the projected water quality in 2035".
Too much fertilizer in the water
The study states: "We can show that with an expected forest loss of up to 80 percent in the Rappbode Vorsperre, the dissolved phosphorus concentrations will increase by 85 percent and the nitrogen concentrations by more than 120 percent within just 15 years," says Kong. This could lead to problematic growth, for example, of more than 80 percent in diatoms and even more than 200 percent in green algae. "Deforestation as an indirect consequence of climate change has a stronger effect on the water quality of dams than direct effects of climate change, such as the increase in water temperature. To that extent, it actually surprised us,” summarizes Kong.
The researchers say that the results for the Rappbodetalsperre can also be transferred to other catchment areas of dams in comparable regions. They make it clear that adjustments will be necessary at various levels in drinking water management. "In the catchment areas of dams, nutrient inputs should be reduced even more than before, reforestation projects that have already started with drought-resistant tree species should be further advanced and waterworks should be adapted to the upcoming developments with selective water extraction strategies," says Rode.
Source: Helmholtz Center for Environmental Research, specialist article: Water Research, doi: 10.1016/j.watres.2022.118721