Prof Dr Tobias Krüger is Professor of Hydrology and Society at the Institute of Geography at Humboldt-Universität zu Berlin. There he heads the Integrative Research Institute on Human-Environment Systems (IRI THESys). In the Einstein Research Unit "Climate and Water under Chance" (CliWaC), his team is investigating the effects of climate change on water resources in the Berlin-Brandenburg region. He talks about this in an interview.
Mr Krüger, Berlin is one of the most water-rich cities in Germany, with around seven per cent of its surface area consisting of water. Brandenburg has more than 3,000 lakes and is the federal state with the largest water surface area. On the other hand, the region has particularly low levels of precipitation. Does this make Berlin the ideal location for your research into hydrology and society?
Prof Dr Tobias Krüger: Yes, the region is predestined for questions on the current topic of increasing drought! It is severely affected by climate change. The Spree, for example, carries little water in summer, and in some places it even flows backwards. This apparent contradiction, that there are an incredible number of lakes and water, but still little precipitation, makes it interesting from a scientific point of view. The question is: What will water availability look like in the future? This also raises important questions from a political and water management perspective that need to be considered. It is scientifically exciting to mediate these apparent contradictions and to exchange ideas with other stakeholders.
Which stakeholders are these?
Krüger: Due to the growing drought, we can research things locally for which we had to travel to other regions of the world just a few years ago. The critical issues that concern the world are now happening on our doorstep. It is now easy to conduct participatory research with other stakeholders on the ground. Our Hydrology and Society department involves water-related actors, citizens who are concerned about such issues or decision-makers at city and state level. We want to do our research together with them. This is more difficult in other regions of the world because you first have to familiarise yourself with the local contexts and spend a long time on the ground. This kind of helicopter science is to be viewed critically; you fly in from a western country and carry out research there. Here, as a university, we have a presence in the region and play a role in decision-making.
How is climate change affecting water resources in the Berlin-Brandenburg region? What insights have you gained?
Krüger: We were able to show that evaporation from open areas and, above all, transpiration from plants, i.e. their release of water into the atmosphere, play a key role in our region. We will continue to experience increasingly warmer temperatures in the future. This will continue to drive evaporation and transpiration. We were able to make it clear that this is a more important factor than decreasing precipitation. In other words, it is primarily desiccation phenomena that cause these droughts, combined with insufficient storage of precipitation.
What research questions arise from this finding?
Krüger: Evaporation and transpiration do not go on indefinitely: there are limits when there is no more water from the soil due to a lack of precipitation. In fact, it is not yet understood how water is replenished from deeper soil layers in our latitudes at the extreme temperatures we are now facing. The current evaporation trend could then level off at higher temperatures.
Another new question is: What happens to the water in the atmosphere? How much is transported eastwards by winds, i.e. how much is lost to the region? How much falls back into the region as precipitation?
How can your research help to manage drought and heavy rainfall, which has also increased significantly in recent years?
Krüger: We are looking at measures that store water in the landscape or in the city. Heavy rain must not be allowed to run off too quickly. It has to be absorbed and stored somewhere so that it is available again in dry periods. These can be artificial basins or renaturalisation measures that give small rivers such as the Panke more space. We can assess which measures are suitable for which part of the city or region so that other problems such as flooding areas are not created. Our results thus confirm the value of the idea of a sponge city. In this way, we provide a scientific basis for policymakers.
Because they can use your results to react to climatic extremes such as lightning droughts, floods and heavy rainfall?
Krüger: On the one hand that, and on the other as a corrective. In current research, I see that the issue of water scarcity is being increasingly mobilised by political actors. In some cases, they are already taking a very territorial view of water resources. This means that we as scientists also have to do a bit of fact-checking so that the issue of water is not arbitrarily instrumentalised politically. Because I don't believe that we have problems that we can't solve if we can just get all the players round the table.
Interview: Vera Görgen