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The recent drought events (e.g. 2022) highlighted the impacts caused by hydrological drought and low-flow events to society and ecosystems. The consequences of low-flow events in recent years emphasizes the urgent need for a structured low-flow risk management. The DryRivers project aims to develop a software-based tool for an effective support of low-flow risk management. The low-flow risk analysis is the core of the supporting tool and will be described in detail in this work.In the field of flood risk applications, scenario-based calculations are often performed. Due to a relative short duration of flood events between a few days to a few weeks and in general negligible hydrological interaction between temporal distant flood events, a clear distinction of such events is quite simple. However, for low-flow risk modelling, the definition of scenarios is considerably more complex due to their long-term development and occurrence. Thus, hydrological conditions from previous years can be essential for the development of a low-flow event. Due to this, the use of long-term continuous time series seems to be more suitable for low-flow risk modelling than a scenario-based approach. This continuous approach has been used in flood risk analysis by Sairam et. al. (2021). In this work it is adapted and extended for low-flow risk analysis.The approach to low-flow risk analysis consists of four basic analyses. These include the meteorological-hydrological analysis, which generates synthetic long-term weather data - using, e.g., a stochastic weather generator - and transforms these weather data into long-term runoff time series. Therefore, a rainfall-runoff model is applied, considering the catchment-specific characteristics. The hydrodynamic analysis quantifies water levels, water temperatures and flow velocities along the river. Core of the analysis is a numerical 1D-river model, which calculates the hydraulic values using runoff time series and river characteristics (e.g., cross sections). The influence of the near-surface groundwater on the river by in-/exfiltration is considered via a bidirectionally coupled 2D-groundwater model. Water temperature is determined in a unidirectionally coupled temperature model. Weather data and hydraulic values are transformed into water temperature within the river. Based on the time series of the hydraulic values the consequences of low-flow are quantified as sum over the considered period within the analysis of consequences. Different categories of low-flow consequences are considered: socioeconomic consequences, e.g., for shipping or industrial water use, as well as ecological consequences for fish and macrozoobenthos. Threshold approaches for quantifying impacts are generally applied in both categories. Finally, in the risk analysis, the low-flow risk is calculated by dividing the damage sums per consequence category with the number of simulated years. This results in an annual low-flow risk, e.g., in €/a. The calculated low-flow risk is an essential basis for a transparent and objective decision support in low-flow risk management.This research is funded within the research framework of WaX (Wasser-Extremereignisse) by the Federal Ministry of Education and Research of Germany. Sairam, N., Brill, F., Sieg, T., Farrag, M., Kellermann, P. and Nguyen, V. D. (2021), Process‐Based Flood Risk Assessment for Germany, Earth's Future 9 (10), DOI: 10.1029/2021EF002259.

Die klimabedingte Zunahme extremer Wetterereignisse ist eine der Herausforderungen in der gegenwärtig durch Krisen geprägten Zeit. Gefahren gehen von großräumigen Hochwasserereignissen und von kleinräumigen Überflutungen innerhalb besiedelter Bereiche aus. Ursache sind ausgeprägte Extremniederschläge. Das Schadpotenzial hängt maßgeblich von den lokalen Bedingungen ab. Ein Starkregen richtet im Flachland mit versickerungsfähigen Böden wesentlich weniger Schäden an als im Bereich eines Kerbtalgewässers, wenn dort die Hochwasserwelle auf bebaute Talbereiche trifft. Auch sind die Folgen einer als urbane Sturzflut bezeichneten Überflutung in dicht besiedelten Räumen in der Regel dramatischer als bei ländlich geprägten Siedlungsstrukturen. Aber nicht nur dem Problem „zu viel Wasser“, sondern auch der zunehmenden Herausforderung „zu wenig Wasser“ muss sich die Wasserwirtschaft stellen. Trockenheit und Hitze führen vor allem in Innenstädten immer häufiger zu Bedingungen, in denen das Leben und Arbeiten zur Belastung wird. Vielerorts sinkende Grundwasserspiegel stellen die Bewirtschaftung natürlicher Wasserressourcen und nicht zuletzt die öffentliche Wasserversorgung in Deutschland vor bislang weitgehend unbekannte Herausforderungen. Einen absoluten Schutz gegen Überflutungen und vor Hitzeperioden gibt es nicht. Wir müssen Vorsorge betreiben, um die Belastungen zu begrenzen. In der letzten Zeit hat dafür der Begriff der „Resilienz“ im wasserwirtschaftlichen Kontext eine besondere Bedeutung gewonnen. Dazu erforderliche Konzepte greifen die Wassertage Münster im Jahr 2023 auf. Zu den Maßnahmen der wasserbewussten Stadtentwicklung zählen beispielsweise die gezielte Versickerung und Verdunstung von Niederschlagswasser oder die Ableitung von Oberflächenabflüssen bei seltenen Starkregen in weniger kritische Bereiche. Thematisiert wird auch der Umgang mit (Ab-)Wasser als Ressource. Hierbei werden die Bedeutung und Chancen der Wasserwiederverwendung betrachtet.

In recent years, the issue of high groundwater levels has caught attention. Unfavorable consequences of high groundwater levels are especially damage to buildings, infrastructure, and the environment. Processes that lead to high groundwater levels are hydrological (heavy or extended rainfall and flood events), or anthropogenic (reduced groundwater extractions, interaction with sewer networks, hydraulic engineering measures, structural interventions in the water balance, and mining activities). Several different map products have been prepared for the information of inhabitants and for planning purposes, and also methods for damage and risk analysis related to high groundwater levels have been developed. Groundwater management measures and structural measures are available to reduce the risk related to high groundwater levels. An operational management system could be combined from existing components, but operational forecasting systems for high groundwater levels are—different to flood forecasting systems—not yet common practice. A better understanding of the processes and the development of integrated approaches for modeling, design, planning, forecasting, and warning, as well as improvement of interdisciplinary collaboration between different organizations, are recommendations for the future. This article is categorized under: Engineering Water > Engineering Water Water and Life > Conservation, Management, and Awareness Science of Water > Hydrological Processes Science of Water > Water Extremes, Pumping water from a basement during the Neiße flood 2010 in Saxony. The clear water indicates that the basement flooding originates from groundwater (photo: Reinhard Schinke).