What if a typical Hamburg drizzle could escalate into a catastrophic flash flood within minutes, overwhelming drainage systems and threatening critical infrastructure? This is the very scenario that Professor Simon Papalexiou, a Greek-German hydrologist, is tackling head-on with the establishment of the Institute for Global Water Security at Hamburg University of Technology (TUHH).
The Imperative of Preparedness: Beyond Daily Forecasts
Papalexiou’s work transcends the realm of daily weather predictions. His focus is on understanding and quantifying the ‘unforeseeable’ – those rare, yet devastating, hydrological extremes such as severe rainfall, floods, and droughts. Traditional models, he argues, excel at predicting averages but fall short when it comes to these critical outliers.
“Risks are often underestimated due to how our brains function; we orient ourselves toward everyday norms,” explains Papalexiou. “But policymakers, urban planners, emergency services, healthcare systems, businesses, and insurers must be prepared for the real dangers.”
Hamburg, for instance, is well-prepared for storm surges and heavy rainfall individually, but not for their simultaneous occurrence. Papalexiou’s institute will not pinpoint the exact timing of such events, but rather simulate their impact. By unleashing ‘artificial storms’ on digital city models, researchers can predict where basements will flood, underpasses will be compromised, and sewer systems will reach their limits.
Stress-Testing the City of Tomorrow
The institute’s approach involves rigorous stress tests for urban infrastructure. “For emergency planning and the resilience of critical infrastructure, it is not enough to rely on the past,” Papalexiou emphasizes. “We must run through all plausible scenarios.”
Existing global climate models, with their coarse resolution of 100 by 100 kilometers, are inadequate for local decision-making. “The entire city of Hamburg fits easily into a single square in such a grid-far too imprecise for sound decision-making,” he notes. A key objective of the new institute is therefore ‘downscaling’ these results to a local level, enabling concrete assessments for individual streets or structures across Germany. This could influence everything from the storm-surge-proof design of new residential towers to insurance premium adjustments based on basement wall thickness.
The Silent Guardians: Hydrology Meets Mathematics
Papalexiou’s work operates behind the scenes, a ‘business-to-business’ model where his findings empower decision-makers. If successful, citizens might never fully grasp why a bridge held during a storm or why grain supplies remained stable during a drought – the unseen triumphs of proactive planning.
His research merges mathematics and hydrology, the science of water on and beneath the Earth’s surface. Hydrologists investigate the ‘what happens next’ after rain falls: its flow into streams, rivers, and sewers; its interaction with soil, lakes, and groundwater; and the frequency and causes of floods, heavy rainfall, and droughts. They also analyze how human activities like urbanization, agriculture, and dam construction alter the water cycle.
Confronting Uncertainty: The End of Illusory Certainty
The catastrophic floods in Germany’s Ahr Valley in 2021 and the dam failure in Derna, Libya, in 2023, which claimed thousands of lives, serve as stark reminders of the dangers of illusory certainty. Papalexiou’s models aim to quantify uncertainty rather than conceal it. “We will have to come to terms with uncertainty. Uncertainty is increasing. If we accept that, we can prepare for it and handle it better,” he advises.
He is driven by the potential of complex statistical systems to create better models. “I am delighted to see the complexity of the world reflected in mathematical formulas. In doing so, I make a small contribution to ensuring that people can live more safely in the future-that cities are better planned and structures endure longer, that fewer people are put at risk. I am proud of that.”
These AI-based models enable researchers to generate high-resolution precipitation fields and better investigate extreme heavy rainfall events and flood risks under changing climate conditions. A simulation of rainstorms using advanced artificial intelligence methods illustrates this approach. (Papalexiou & Mamalakis (2026), Journal of Hydrology. https://doi.org/10.1016/j.jhydrol.2025.134689)
An Optimist’s Vision for Hamburg’s Future
Despite his daily engagement with disaster scenarios, Papalexiou remains an optimist. Having returned to Europe after years abroad, he appreciates the creative freedom of the European university system, allowing him to focus on truly impactful work. He and his family have recently moved to Hamburg, a city he hopes will embody the resilience his research strives for.
In a future shaped by his work, when heavy rain falls, Hamburg might respond with pre-planned precision: retention basins opening beneath HafenCity, pumps controlled by thousands of calculated scenarios, and streets designed to temporarily manage water. A warning light at an underpass might flash: “Closed – risk of flooding!” The news would then report: “Heavy rain front over Hamburg – traffic disruptions, no major damage.” Such an outcome would be a testament to the proactive measures taken based on Papalexiou’s insights, ensuring the city’s safety without citizens ever realizing how close they came to disaster without these new ways of thinking.
This research project contributes to UN Sustainable Development Goal No. 9, which focuses on industry, innovation, and infrastructure.
