Interview
“We need to consider both too much and too little water together”
Heavy rainfall event in Meiningen in 2018, resulting in flooding. Photo: Helge Busch-Paulick, Wikipedia (CC-BY-SA 3.0)
Heat, drought, low water levels, and warm rivers show just how much pressure Germany’s water balance is under. KIT researchers Peter Knippertz and Harald Kunstmann explain why heavy rainfall, flooding, and drought must be considered together - and how Helmholtz can contribute to this effort.
Mr. Knippertz, Mr. Kunstmann, what does this summer tell us about the state of Germany’s water balance?
Peter Knippertz: First of all, it shows how closely weather patterns and the water cycle are linked. Prolonged periods of high pressure in the summer often bring little precipitation and high temperatures. Soils dry out, rivers carry less water, trees come under stress, and agriculture also feels the consequences. If very hot air is subsequently lifted - for example, by a cold front - particularly severe thunderstorms can develop. When heavy rain then hits parched soil, the water drains away less effectively. Drought and heavy rain are therefore not entirely separate phenomena.
Harald Kunstmann: To me, the situation shows above all that we need to manage water differently. During floods, the goal is to get the water out of the area as quickly as possible to prevent damage. Weeks or months later, however, that very same water may be needed for irrigation, cooling, or ecosystems. Historically, flood protection and drought prevention were often considered separately. That’s no longer enough. We need to retain more water in the landscape - for example, through “sponge city” concepts, restored wetlands, or increased infiltration. We’re still in the early stages of this, though.
When people talk about hydrometeorological extreme events, it sounds a bit unwieldy at first. What does this term actually mean?
Peter Knippertz: The term combines meteorology and hydrology. It refers to extremes in which weather processes and the water cycle interact. On the dry side, these include dry spells, droughts, and low water levels; on the wet side, they include heavy rain, flash floods, and high water levels. Some events occur on a very small scale and unfold rapidly, such as when thunderstorms trigger flash floods in small watersheds. Others affect larger regions or rivers, such as when a low-pressure system slowly moves very humid air masses over a region, resulting in heavy precipitation.
Prof. Dr. Peter Knippertz heads the Atmospheric Dynamics Research Group at the Institute for Meteorology and Climate Research—Troposphere Research (IMKTRO) at the Karlsruhe Institute of Technology (KIT). Photo: KIT
Where has research made progress in recent years, and where do significant uncertainties remain?
Peter Knippertz: Today, we have a better understanding of how large, prolonged heavy rainfall events form. Take the Ahr flood, for example: The seas surrounding Europe - especially the Mediterranean and the Baltic Sea - were very warm at the time. This allowed a great deal of moisture to enter the air. A low-pressure system then pushed this moist air toward the low mountain ranges, where it was lifted and triggered enormous amounts of precipitation. Such initial conditions are likely to occur more frequently in the future because high sea surface temperatures are becoming more probable. In contrast, there remains a great deal of uncertainty regarding small-scale heavy rain events. Such thunderstorms are highly dynamic, release a great deal of energy, and change rapidly in both space and time. We are not yet always able to observe them well enough.
Mr. Kunstmann, what happens on the other end of the water extremes spectrum: When does prolonged drought become a hydrological problem?
Harald Kunstmann: First, there is a prolonged lack of precipitation. Hydrologically, it becomes a problem when soil moisture, groundwater, lakes, and rivers fall significantly below normal levels. It’s not just the lack of rain that plays a role here, but also temperature. Heat increases evaporation, which further depletes groundwater, soils, and water bodies of water. If irrigation or other management measures then prove insufficient, this can develop into an agricultural drought that affects crop yields. It’s also important to note that these systems respond at different rates. Rivers and near-surface reservoirs can change relatively quickly after rain. Groundwater, on the other hand, is an inert system. Once it reaches a low level, it can take months, years, or even longer to recover.
The HydroExtremes project focuses on the entire chain of weather, water balance, and impact. Where is this process chain still particularly uncertain today?
Peter Knippertz: As I mentioned earlier, from a meteorological standpoint, accurately forecasting thunderstorms that can trigger flash floods remains a major challenge. It’s not just a matter of whether they’re possible, but also of when and where they’ll actually occur and how severe they’ll be. To address this, the German Weather Service uses ensemble forecasts - that is, multiple model runs that simulate different scenarios. However, these ensembles do not always adequately capture the actual uncertainty. Sometimes what ultimately happens falls outside the calculated possibilities. To improve in this area, we need more precise observations on small scales, such as of air currents that can trigger or intensify thunderstorms.
Harald Kunstmann: But the forecast doesn’t stop at precipitation. The key is to better integrate the entire chain of processes: from the raindrops in the atmosphere to the water level that ultimately forms on a street, in a parking lot, or in an underground parking garage. We’re improving in individual areas. But these individual segments still need to be linked more closely, like links in a chain. This includes meteorology, hydrology, and hydraulics - that is, the translation into specific water levels and flood areas. That’s precisely where major uncertainties still lie.
Prof. Dr. Harald Kunstmann is deputy director of the Institute for Meteorology and Climate Research—Atmospheric Environmental Research (IMKIFU), head of the Regional Climate Systems Division at the KIT Alpine Campus in Garmisch-Partenkirchen, holds the Chair in Regional Climate and Hydrology at the University of Augsburg, and is also the founding director of the Center for Climate Resilience there. Photo: KIT/Markus Breig
Is that where Helmholtz can play to its strengths?
Harald Kunstmann: Yes, because Helmholtz possesses a wide range of expertise along this process chain. The links in the chain are, so to speak, already in the workshop; they just aren’t all connected yet. Continuity is also a major advantage. We can continue to refine methods, models, and observation systems over many years and maintain our expertise. This is often more difficult at universities because many projects end after just a few years, and then you have to start over from scratch. This long-term commitment is very important for increasingly complex models and measurement systems.
Peter Knippertz: Then there’s the infrastructure. Helmholtz can generate large amounts of new and novel data, for example through field measurements. This is particularly important right now because AI methods are opening up new possibilities. When a lot of high-quality data is available, it can be used to develop new forecasting tools. At KIT, for example, we’re trying to consolidate and standardize data from state flood centers to train flood prediction models. Helmholtz can provide data, ideas, and scientific insights here that authorities can later put into practice.
In your view, what is the most important message from HydroExtremes?
Harald Kunstmann: We need to consider both too much and too little water together. This affects a great many stakeholders with differing interests. That’s why, in addition to scientific and technical expertise, we also need social science knowledge about how such changes can be implemented.
Peter Knippertz: And we need to start talking with the authorities earlier. It’s crucial that solutions can actually be used in practice. So research must consider from the very beginning what can ultimately be implemented.
HydroExtremes
In the HydroExtremes project, the Helmholtz Forum Earth and Environment brings together the expertise of several Helmholtz Centers on hydrometeorological extreme events. The focus is on the entire process chain - from weather and precipitation through soil, groundwater, and rivers to floods, droughts, and their consequences. The goal is to synthesize the current state of research, identify open questions, and lay the groundwork for better preparedness and adaptation.
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