The consequences of climate change are increasing and we have been able to experience them in our own flesh. Heat waves, increasingly intense, prolonged and frequent, floods, prolonged droughts and the virulence of forest fires are no strangers to us. Another consequence of climate change, this time indirect, is the disappearance of forests , which can affect the quality of the water we drink.
Forests play a very important role in the water cycle, filtering it and fixing nutrients. The fewer nutrients the water in a reservoir has, the easier it will be to make it drinkable. This is so because it hinders the growth of algae and consequently the treatment that must be given to be able to drink it is simpler and more profitable. However, there are places whose forests have been severely weakened, even losing 50% of the trees that make them up.
This is the case of the Harz region in Germany, affected by long periods of drought between 2015 and 2020. The Rappbode reservoir is located there. The drought has taken its toll on the trees and parasites like bark beetles have found it easier to spread. Parasites that have further damaged trees to the point of quickly killing them. “In the last four years, the Rappbode catchment area, characterized by conifers, mainly spruce, has lost more than 50% of its forest ,” says UFZ hydrologist and one of the study’s authors, Professor Michael Rhodes. “This massive retreat of the forest is advancing rapidly and is dramatic. This will have consequences for the drinking water reservoir.” The Rappbode is the largest drinking water reservoir in Germany. It provides drinking water to approximately one million people.
Seeing that the decline of the forests can cause difficulties when it comes to treating the water in the reservoirs and making it drinkable, a team of researchers from the Helmholtz Center for Environmental Research – UFZ has investigated the effects of climate-induced deforestation on the reservoir water quality. The study was based on data from the network of environmental observatories TERENO (Terrestrial Environmental Observatories), in which the UFZ participates with the Harz Lowland Observatory/Central Germany.
Scientists have accessed data from more than 10 years ago . They have also used data from the international project ISIMIP (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 in the catchment area,” explains Dr. Xiangzhen Kong, who was also involved in the study. “The resulting data was then processed into a reservoir ecosystem model with which we were able to determine the effects of different deforestation scenarios on projected water quality in 2035. “
The Rappbode reservoir receives water from three different hydrographic basins, two of which have been included in the study: Hassel and Rappbode. The first is characterized by agriculture and the second is forestry, at least before the spruce forests died.
Before flowing into the reservoir, the water reaches a dam. The influence of agriculture makes the nutrient content in the water of the Hassel Dam much higher than that of the Rappbode Dam. “We have been able to show that, for a projected deforestation of up to 80%, the previous Rappbode Dam will experience an 85% increase in dissolved phosphorus concentration and a more than 120% increase in nitrogen concentration in just 15 years. The previous Rappbode dam will thus achieve almost the same nutrient levels as the previous Hassel dam,” says Kong. The result will be an increase of more than 80% of diatoms (a group of algae) and more than 200% of green algae in Rappbode Dam.
All this highlights the need to carry out new measures in terms of drinking water management. The researchers say it will be necessary to further reduce the amount of nutrients going into the reservoir water. To this end, they propose to continue reforesting the area with trees that resist drought and advancing in strategies for the selective elimination of water .
“The death of forests, as an indirect consequence of climate change, has a more pronounced effect on the water quality of reservoirs than the direct effects of climate change, such as high water temperatures. In fact, we were surprised by the magnitude of this effect,” says Kong.
Referencia: Kong, X., Ghaffar, S. et. al. 2022. Reservoir water quality deterioration due to deforestation emphasizes the indirect effects of global change. Water Research. DOI: 10.1016/j.watres.2022.118721
