Federal Institute of Hydrology

Water plays a central connecting role within the climate system, linking meteorological, hydrological, and earth system processes with societal dimensions such as water resource demands and risks associated with pollution, droughts, and floods. Global water challenges are addressed through multiple international agendas related to sustainable development, climate change, biodiversity, and water security. While these agendas share broadly aligned objectives, they differ in scope, scale, and modes of operationalization. Together, they shape how scientific knowledge is mobilized, policies are formulated, and actions are implemented. As the 2030 Agenda approaches its conclusion, there is a growing need to review and map global water agendas in order to better understand their interactions and support more coherent responses to complex water challenges.Rather than viewing global water agendas as parallel and independent efforts, they can be understood as interconnected learning pathways through which shared objectives, knowledge, and practical experience evolve over time. From this perspective, synergies emerge across these learning pathways through reflection, coordination, and exchange between science, policy, and practice. Clarifying how such synergies can be recognized, supported, and strengthened is therefore essential for advancing more integrated and impactful responses to global water challenges.The Strategic UN Synergy Working Group (SUN) operates within the IAHS HELPING Science for Solutions Decade (2023–2032) and aims to strengthen the contribution of hydrological science to international policy processes and practical implementation programmes. Guided by the HELPING paradigm, SUN facilitates bottom-up engagement, open science, and co-creation principles to support learning across scales and the translation of hydrological knowledge into policy and action.This contribution introduces the vision, structure, and core activities of the SUN Working Group, with a focus on understanding global water agendas and supporting synergies through a science–policy–practice approach. SUN builds on the understanding of global water agendas as interconnected learning pathways, and we will illustrate how coordinated learning pathways can help advance more coherent, integrated, and future-oriented global water agenda.

Drought occurrences have become more frequent across all continents in recent years, leading to greater emphasis on understanding their impacts on water resources and socioeconomic conditions. Despite the existence of several global drought monitoring systems, a comprehensive multisectoral approach, that integrates the impact on water, agriculture, ecosystems and society, is still lacking. We therefore present a multi-sectoral global drought hazard monitoring dataset (histMDH) for the period of 1981-2020 covering five key sectors: water supply, riverine and non-agricultural land ecosystems, and both rainfed and irrigated agriculture. With a period of 40 years coverage, histMDH is suitable to be used as the baseline/reference period for a near real-time monitoring and forecasting system, part of which will be used in an operational system in future. The dataset is derived from a modelling chain using the ERA5 reanalysis data (produced by the European Centre for Medium-Range Weather Forecasts) as climate forcing for two global models: Global Crop Water Model (GCWM) and Global Hydrological Model (WaterGAP) to generate a suite of multi-sectoral drought hazard indicators (DHI). The resulting gridded monthly dataset comprises eleven DHIs (two meteorological, seven hydrological, and two agricultural), spanning 1981–2020. The DHIs defined can be used to identify droughts across different sectors and consequently define their characteristics and intersectoral impacts. The suitability of the DHIs for drought monitoring was assessed using multiple independent data sources at global and regional scales. As an open-access dataset, histMDH provides a critical baseline for near real-time drought hazard monitoring and forecasting within operational systems. It offers valuable support for decision-making in water management, agriculture, and food and water security monitoring. Furthermore, the spatio-temporal variability of DHIs at global and regional scales enables the identification of drought-prone regions, allowing to mitigate drought impacts and transition to more resilient agricultural, ecological and water supply systems.

• Systematic mapping of common GI across 112 UK cities identified 80 configurations. • GI costs range from £2.5/m² for grass to £248/m² for combination with green walls. • Household GI combinations covered areas from 4 to 80 m². • Economic benefits can reach ∼£800 per m² yearly with efficient GI configurations. • DIY scores show cost return is higher for household GI than for street scale. While extensive research has documented the environmental benefits of green infrastructure (GI), do-it-yourself (DIY) approaches that empower individuals and communities to implement small-scale green solutions have received limited attention despite their potential to democratise urban greening and foster community engagement. This study aims to systematically characterise DIY GI implementation across UK cities and analyse the costs and benefits of GI establishment. We conducted mapping of GI configurations across major UK cities using Google street view imagery and field observations, identifying 30 street-scale and 50 household-scale GI scenarios, complemented by an extensive literature review examining small-scale GI interventions. This focus on household and streetside GI provides insights into small-scale interventions, although larger spaces such as urban forests, parks and grasslands also provide benefits, they are beyond the scope of this study. A five-level scoring framework was developed for costs, space requirements, expertise levels, maintenance demands, and cost-benefits to design DIY guidance cards for GI scenarios. Our findings reveal diversity in GI adoption, with street trees and basic grass combinations dominating street-scale implementations (20.8 % each), while household-scale approaches show remarkable variety ranging from simple grass-only configurations (18.5 %) to complex multi-feature systems. The analysis identified a linear relationship between higher GI establishment costs and increased maintenance costs, despite greater variation in the latter. The detailed DIY score reveals that household-scale combinations have greater variation in cost and higher potential benefits compared with street-scale interventions, which remain clustered at lower scores despite higher space demands. The DIY framework enables local authorities and households to make informed GI decisions, addressing key implementation barriers. Aligned with UK policies for GI, the current framework can be further enhanced through interactive platforms, planting schemes, and added metrics for biodiversity and climate adaptation.

High-resolution real-time monitoring gains increasing importance in environmental monitoring. We employed and evaluated three real-time monitoring techniques to monitor anions at the river Rhine in Koblenz, Germany. These are a sensor for nitrate in situ and ex situ, a colorimetric device for nitrate and nitrite, and an online ion chromatography (IC) for fluoride, bromide, chloride, nitrate, nitrite, phosphate, and sulfate. The optical sensor performed well ex situ while in situ, a higher maintenance effort was needed due to fouling processes. Colorimetry failed to analyze and notify when solutions fouled and required a very high amount of maintenance to reliably analyze nitrate and nitrite. The online–IC was feasible for analyzing seven anions with a low amount of maintenance once the required infrastructure for the instrument was provided. The online high-resolution IC data was compared with conventional monitoring data with, e.g., a routine interval of two weeks, which was shown to fail to capture peak concentrations during high discharge events. Additionally, to monitor short-term events, like the shortest registered peaks of 5 h, monitoring intervals of at least 2 h at the river Rhine are needed. By extending the IC time series to 2.5 years supported by principal component analysis, a dendrogram clustering, and event analysis by QC-diagrams, two major groups of anions were visible. The first group includes discharge-driven anions such as chloride, bromide, sulfate, and fluoride, with fluoride being additionally influenced by the equilibrium precipitation of fluorite. The second group consists of seasonal-driven anions like nitrate, nitrite, and phosphate, with phosphate being also affected by short-term events and hence, remains to be an important indicator in online monitoring setups. To get a complete picture, anion analyses can be combined with, e.g., element or non-target analyses.Supplementary InformationThe online version contains supplementary material available at 10.1007/s10661-025-14954-y.

Abstract. The hyper-turbid Ems estuary has undergone extensive channel deepening since the 1980s resulting in distinct tidal asymmetry and substantial sediment accumulation which led to the occurrence of a fluid mud layer. This might cover up to 60 % of the water column, creating density-driven stratification that significantly affects hydrodynamics, ecology, and navigability. During a six-week monitoring period we investigated fluid mud dynamics with the focus on occurrence, thickness, density, and driving processes to obtain insights about formation and break-up of the fluid mud layer. We identified two superimposed recurring cycles of fluid mud occurrence. On a shorter timescale, we observed distinct differences over the semidiurnal tide while on a longer timescale fluid mud occurrence shows a distinct spring-neap tide relation. We conducted dedicated measurement campaigns around spring and neap tide to further investigate spring-neap variability of fluid mud occurrence. One main finding of those campaigns is that during neap tide, the majority of the water column is covered by a dense fluid mud layer, thus reducing the hydrodynamic cross-section. This leads to increased current velocity and entrainment at the lutocline, and reduced stability of the fluid mud layer. In contrast during spring tide, the hydrodynamic cross-section remains wider, leading to less friction at the lutocline and thus enhances stability of the fluid mud layer and thus longer persistence of fluid mud.

Reversed-phase liquid chromatography coupled to high-resolution mass spectrometry (RP-LC-HRMS) is the standard for nontarget screening (NTS) of environmental samples but lacks retention of highly polar contaminants. We compared 12 chromatographic methods across four platforms, RP-LC, anion chromatography (IC), supercritical fluid chromatography (SFC), and hydrophilic interaction chromatography (HILIC), using 127 environmentally relevant compounds (logDpH7.4 -5.6 to 6.6). Compounds were analyzed in solvent and for a polar subset, in groundwater enriched by vacuum evaporation. Data were collected across four laboratories using 5 RP-LC-, 3 HILIC-, 2 SFC-, and 2 IC-HRMS setups. Feature detection with standard tools yielded more false negatives for SFC and IC. To enable a fair and method-agnostic comparison, an extracted ion chromatogram (EIC)-based workflow was used. Of the 127 compounds, 125 were detected by at least one platform. For logDpH7.4 > 0, RP-LC covered ∼90%, followed by SFC (∼70%), while IC and HILIC each covered <30%. For very polar compounds (logDpH7.4 < 0), coverage dropped across all platforms. SFC and HILIC detected up to 60% of polar analytes; IC performed better in negative ionization mode, consistent with anion-exchange separation. Detection frequency declined with polarity, reflecting analytical limitations. Combining RP-LC with either SFC or HILIC increased coverage to 94%. In spiked groundwater, coverage was lower (73%) due to matrix effects and losses during vacuum enrichment. Peak widths were narrowest for SFC (∼2.5 s) and RP-LC (∼4 s) and broadest for HILIC (∼7 s) and IC (∼17 s). Retention times showed limited cross-platform correlation while ionization efficiency was consistent, except for SFC. As no single method provided full coverage, combining RP-LC with one complementary platform (SFC, HILIC, or IC) is required to extend chemical space in environmental NTS.

Abstract Tidal reeds provide essential coastal protection by shielding the shore from the effects of waves and currents, reducing soil erosion or even enabling sediment accretion. However, these environments face increasing threats from human activities and climate change. This study focuses on the Elbe estuary in Germany, investigating factors that influence tidal reed expansion and recession. We investigated the dynamics of three shore sections, comparing reed‐dominated and grassland‐dominated shore sections, and assessed their ability to attenuate waves and mitigate shoreline erosion. Our findings reveal significant differences in shoreline dynamics between the sites, influenced by factors such as hydrodynamic stress, vegetation cover, and sediment composition. In particular, we highlight how tidal reeds contribute to shore stabilization through wave attenuation and the strengthening effects of their root systems during periods of reduced plant biomass in winter. However, the effectiveness of reeds in shoreline protection depends on the level of external stress, particularly from storm surges, with areas exposed to higher hydrodynamic forces showing greater erosion. The study underscores the importance of maintaining tidal reeds for resilient coastal protection, while also acknowledging that external stress factors sometimes require supplementary measures to ensure long‐term shoreline stability. Our results contribute to a deeper understanding of estuarine shore dynamics and offer insights for future management strategies.