Abstract: EU-Hydro is a hydrographic reference dataset and part of the Copernicus Land Monitoring Service (CLMS) portfolio, implemented by the European Environment Agency (EEA). It offers detailed information on the geographical distribution and spatial characteristics of water resources throughout Europe, such as river networks, surface water bodies and watersheds. EU-Hydro was initially developed in 2012, with subsequent updates aimed at improving data accuracy and network topology. However, inconsistencies remained, allowing EU-Hydro to be used for mapping applications, whereas its use for hydrological modelling remains limited. It is currently being updated to produce an improved and upgraded version of this unique European reference dataset. Highlighting the importance of water mapping and modelling, the new version of EU-Hydro (EU-Hydro 2.0) shall tackle the requirements of a modern reference product within the pan-European hydrological domain, serving various use cases, such as: supporting water quality and availability analysis; runoff modelling or flood modelling and prediction; environmental assessments related to river connectivity or the evaluation of anthropogenic impacts. The coastline can serve as input for analytical purposes for various applications. Moreover, policy areas such as nature restoration and climate adaptation can be tackled, all with the goal to strengthen water resilience across Europe. The EU-Hydro 2.0 will build upon a latest generation Digital Elevation Model (DEM) to provide highly detailed and high-quality topographic input data: the Copernicus DEM, a pan-European DEM available at 10m resolution, based on the TanDEM-X mission, supported by the Copernicus DEM at 30m resolution for catchments upstream and downstream that flow in and out of the EEA38 +UK area (EU27 + European Free Trade Association (EFTA) + Western Balkans + Turkey + UK). The production of EU-Hydro 2.0 will involve the best possible ancillary data of hydrography, land cover, and infrastructure to allow seamless integration into the DEM editing process, as well as VHR satellite data for quality control and validation. The product suite consists of eight main layers: The three main raster products are the hydrologically conditioned DEM (Hydro-DEM), the Flow Direction (Hydro-DIR) and the Flow Accumulation (Hydro-ACC) maps, supported by additional raster layers for expert hydrological use. The five vector products are the river network (Hydro-NET), water bodies (Hydro-WBO), basins and sub-watersheds (Hydro-BAS), a product on artificial hydrographic structures (Hydro-ART) and a coastline (Hydro-COAST). First, the process of DEM editing and hydro-conditioning involves refining the DEM to ensure it accurately represents the water flow and natural hydrological features. This includes correcting artefacts that interfere with flow connectivity, such as bridges and dams, filling sinks in the DEM that are caused by inherent uncertainties, and adjusting elevation data to create a hydrologically consistent surface, which is critical for accurate water flow analysis and watershed delineation. In particular, novel methods are employed to remove noise and distortions from the DEM due to vegetation cover and urban build-up, to enforce flow paths using high-resolution cartographic layers of water surfaces, rivers and lakes, and to centre drainage lines in the middle of larger water bodies. In a next step, the raster layers (i.e., the flow direction and accumulation maps as well as further advanced hydrological layers) are derived from the hydrologically conditioned DEM surface, and subsequently, the vector layers Hydro-NET and Hydro-BAS are extracted from them. Ancillary data are needed to generate Hydro-WBO, Hydro-COAST and Hydro-ART, which can only be partially derived from the Hydro-DEM. To ensure a homogeneous approach across Europe regardless of geographical differences, and to find the best possible methodologies, the greatest expected challenges are a) the correct delineation of river networks and watershed boundaries in regions with mostly flat terrain, low topographic variation, and dense vegetation cover which affects DEM accuracy, such as large floodplains; b) the correct interpretation of flow topology in highly modified landscapes such as urban or irrigated areas where artificial canals can dominate over elevation-derived flow paths; and c) the correct detection and interpretation of special flow features such as inland depressions, underground flow connections in karst areas, or the complex structures of deltaic systems. Furthermore, inconsistencies among and within European countries related to the quality and completeness of available ancillary datasets used to improve the river and watershed delineations may introduce some regional differences in achievable accuracies. The derived raster layers will be a significant enhancement and novelty to EU-Hydro 2.0, alongside other key additions such as hierarchically nested watersheds, an updated coastline dataset, and detailed maps of water bodies and artificial structures, i.e., the vector products, all integrated within a topologically consistent river network. All layers will be interrelated, scalable and logically consistent. The approach aims at transparency and automation to-the-extent-possible, supported by manual corrections where needed to increase quality and meet user requirements. This will ensure efficient and reproducible data processing and facilitate further updates of EU-Hydro into the future. The pan-European production is targeted to be finalized by summer 2026. The upgraded EU-Hydro 2.0 suite of products will constitute a harmonized, homogeneous and consistent reference dataset for Europe. It will provide a new era not only for water mapping in the framework of CLMS, further Copernicus Services and other fields, but also target hydrological modelling, hydrologic risk assessments, climate change studies, water resource management, environmental protection strategies, and infrastructure planning, hence supporting the implementation of the EU Biodiversity Strategy, in particular the Nature Restoration Law. Its potentially crucial role for society is further emphasised by the rising frequency of natural disasters, such as droughts and floods, the latter being one of the use cases for modelling. With its high-quality, free, and openly accessible data, EU-Hydro 2.0 will help address these challenges, which urgently call for enhanced water resilience in Europe.
Snæfellsjökull and Snæfellsnes Peninsula in west Iceland. Photo by USGS.
