Although the hydrological impacts of land use changes are well studied, few datasets comprehensively capture the influence of land management on hydrochemical processes and solute fluxes. The long-term Wüstebach catchment experiment within the TERENO network (TERrestrial Environmental Observatories) provides a unique infrastructure for monitoring key water balance components, numerous anions and cations, as well as spatiotemporal soil moisture—both before and after partial deforestation and subsequent forest management measures such as thinning and underplanting.We present long-term hydrochemical observations, including macro- and micronutrients, dissolved aluminum, and dissolved organic carbon, collected three years before and thirteen years after deforestation. Hourly concentrations and fluxes were estimated using the R package LOADFLEX. Predicted nitrate concentrations were compared with high-resolution reference data to select the optimal modeling approach. Comparable flux data were determined for a neighboring reference catchment with similar characteristics but without clear-cutting, enabling the isolation of deforestation and reforestation effects on nutrient cycling and transport.Using flux data from both catchments, we applied a Before–After–Control–Impact (BACI) framework to quantify hydrochemical responses and feedbacks to forest management. Three phases were distinguished: pre-deforestation, the first two years after deforestation, and a later post-deforestation phase (three years after). The BACI analysis revealed distinct short- and long-term responses in solute fluxes, with the strongest effects observed for NO₃⁻, dissolved organic carbon (DOC), and Fe. Notably, fluxes during the two-year period immediately following deforestation differed significantly from both the pre-deforestation phase and the later post-deforestation phase, indicating a pronounced but transient disturbance effect. This dataset offers valuable opportunities to investigate the long-term impacts of deforestation and reforestation on hydrochemical fluxes under varying climatic conditions. Bogena, H.R., F. Herrmann, A. Lücke, T. Pütz and H. Vereecken (2025): Long-term hourly stream-water flux data to study the effects of forest management on solute transport processes at the catchment scale. Earth Syst. Sci. Data. 17: 6965–6992. DOI: 10.5194/essd-17-6965-2025

Climate change is altering hydrological processes and water availability, requiring robust long-term analyses based on consistent data and models. In the article, the main components of the hydrological water balance––precipitation, evapotranspiration, and runoff––in Slovenia for the period 1972–2024 are analysed. The analysis is based on results from a verified national-scale mGROWA water balance model. The objective of study is to identify trends that could be relevant for water resources management in a changing climate. A key finding is that evapotranspiration has increased significantly in large parts of the country. However, above-average precipitation in recent years has so far compensated for this increase, preventing a country-wide decline in average runoff. Finally, a hydro-climatological trend indicator is proposed as a future element for monitoring the water balance components in Slovenia.

A substantial body of knowledge concerning the functioning of catchments has been derived from the observation and quantification of solute and suspended matter fluxes in rivers. The Wüstebach catchment is a hydrological observatory that is part of the German TERENO (Terrestrial Environmental Observatories) network. In 2013, the Eifel National Park undertook a partial deforestation of the spruce forest with the objective of facilitating the regrowth of a natural forest. This data paper presents 16 years of estimated hourly stream-water flux data of nine continuously monitored macro- and micronutrients, as well as dissolved ionic aluminum and dissolved organic carbon (DOC), along with the measured solute concentrations and discharge rates observed in the Wüstebach catchment (from 2010 to 2024). To estimate hourly stream-water fluxes from weekly manual grab samples and event autosampler data, we employed the R software package LOADFLEX, which implements a number of solute prediction methods, including regressions, interpolations, the period-weighted approach, and the more recently developed composite method. A comparison of the predicted nitrate concentrations with hourly nitrate reference data was conducted to assess the optimal prediction approach for the Wüstebach catchment. The analysis showed that the composite model is best suited to calculate the nitrate fluxes. Accordingly, this model was selected to calculate the fluxes of all considered macro- and micronutrients, dissolved aluminum and DOC. Flux data were compiled in the same way for a neighboring reference catchment with similar characteristics but without clear-cutting, in order to identify the effects of deforestation and afforestation on the cycling and transport of nutrients. We anticipate that this comprehensive data set will facilitate new insights into the influence of deforestation and afforestation on solute fluxes at the catchment scale. The dataset, entitled “Wüstebach data paper: Long-term hourly solute flux data 2010–2024”, is shared via Forschungszentrum Jülich: https://doi.org/10.26165/JUELICH-DATA/AKAMNQ (Bogena and Herrmann, 2025).

As climate change brings about hotter and often drier summers, an improved understanding of how irrigation requirements vary according to climatic conditions is of increasing importance. Within Germany, temperate conditions have historically enabled most agriculture to be supplied solely by green water, but recent crop yield reductions and crop failures have demonstrated its increased vulnerability to climatic conditions. The raster-based mGROWA hydrological water balance model was implemented over all agricultural areas in Germany for the period 1961–2020 at a high spatial (200 m) and temporal (daily) resolution. Grid-cells were each assigned one of 10 major crop classes, which account for 86.7 % of all agricultural areas in Germany, and effectively all irrigated areas. Using crop-specific irrigation rules that reflect actual practices, irrigation requirements were simulated for all crop areas. To investigate the relationship between climatic water balance over the crop growing season and irrigation requirements, the simulated annual irrigation requirements were compared with the standardised precipitation-evapotranspiration index (SPEI-6), calculated at the end of September. Through this comparison, irrigation requirements could be characterised for near-normal and dry conditions, and results were aggregated to the district level. Additionally, using district-level data on the areas with irrigation infrastructure, the actual water used for irrigation was estimated. The results highlight marked increases in irrigation requirements in dry conditions compared to near-normal conditions (median increase of 72 %), which are more pronounced over crops in silty soils than in sandy soils. The results also demonstrate how the increased irrigation requirements in dry years are in many cases higher than what is suggested by guidelines for irrigation management in Germany. This study provides important information for actors related to the agricultural sector and water management and is based on a robust and transferable framework to quantify how irrigation requirements vary according to climatic variability and local soil conditions.

<p>Obtaining Soil Moisture Content (SMC) over large scales is of key importance in several environmental and agricultural applications especially in the context of climate change and transition to digital farming. Remote sensing (RS) has a demonstrated capability in retrieving SMC over large areas with several operational products already available at different spatiotemporal resolutions. At the same time, cosmic-ray neutron sensing is a recently emerged approach in retrieving high temporal resolution SMC at intermediate spatial scales. The present study conducts an intercomparison between different RS-based soil moisture products, daily SMC retrievals from a cosmic-ray neutron sensor (CRNS) station and a network of in situ SoilNet wireless sensors installed at the Pinios Hydrologic Observatory ILTER site in central Greece for a time period of 2018-2019. The RS-based soil moisture products included herein are from NASA’s Soil Moisture Active Passive (SMAP) and Metop-A/B Advanced Scatterometer (ASCAT) satellite missions. The methodological workflow adopted includes standardized validation procedures employing a series of statistical measures to quantify the agreement between the different RS-based soil moisture products, CRNS-based SMC and the SoilNet ground truth data. Our study results contribute towards global efforts aiming at exploiting CRNS data in the context of soil moisture retrievals and their potential synergies with RS-based products. Furthermore, our findings provide valuable insights into assessing the capability of CRNS at retrieving more accurate SMC estimates at arid and semi-arid environments such as those found in the Mediterranean basin, while supporting also ongoing global validation efforts.</p> <p><strong>Keywords: </strong>Cosmic Ray Neutron Sensors; SMAP; ASCAT; SoilNet; Soil Moisture Content</p>

In many temperate regions globally, sufficient precipitation and moderate temperatures have meant that green water has sufficed for agriculture. However, the effects of climate change demonstrate that additional crop water is now more frequently required in many of these areas, particularly for dry summer years, with irrigation demands expected to continue increasing. In Germany, this effect has become noticeable over previous years, exemplified by the reduced crop yields in the recent summer droughts of 2018 and 2020.Our study, performed within the scope of the WADKlim project, identifies critical hotspots for water stress through high-resolution hydrological modelling and statistical analyses to determine groundwater recharge and theoretical irrigation requirements from now until 2100. We set up and calibrated the mGROWA hydrological model over a historical period (1961-2020) at a high spatial (100 m) and temporal (daily) resolution. The calibrated model was then run until 2100 for three climate scenarios (1 x RCP2.6; 2 x RCP8.5), which were selected as a stress test for the system. As model outputs, we derived the spatio-temporal patterns of groundwater recharge as well as crop water requirements, through the application of irrigation rules typical for Germany and accounting for the spatial distribution of different crop types. We converted the theoretical crop water requirements into requirements only for areas that are equipped for irrigation, incorporating multiple scenarios for the rate at which irrigation infrastructure could expand in Germany.Our results demonstrate the large spatial and interannual variations in irrigation demands throughout Germany. We quantify how the multiplicative effect of warmer and drier summers in combination with increased areas equipped for irrigation is expected to strain water resources in the future. For example, we estimate that mean annual irrigation demands in Germany could increase by as much as 700% by 2075-2100, considering the “worst-case” scenario of climate projection and increase in irrigated areas. Regarding groundwater availability, owing to the expected increase in winter precipitation in Germany, our modelling results show pronounced regional variations in whether or not annual groundwater recharge is expected to increase in the future. Finally, we included estimates of other water requirements and aggregated the results to determine overall water demands at the district level and calculate ratios of water use to groundwater recharge per district. Our results highlight the hotspots in Germany where water stress is expected to increase the most throughout the 21st century, which could likely lead to conflict between different water users (agricultural, industry, public supply).Determining the spatio-temporal characteristics of how water stress will change requires comprehensive assessments of water availability, crop water requirements, areas equipped for irrigation infrastructure, and other water uses. In addition, the large variability in climate projections means that results from such assessments provide large ranges of expected water stress conditions. We have developed and tested a comprehensive methodology for identifying and mapping water hotspots, which we implemented for Germany using three climate projections. Our methodology is transferable to similar (data-rich) regions, and can also be applied for a complete ensemble of climate projections.

Robust assessments of variations in freshwater availability are essential for current and future water resource management in the Pinios River Basin (PRB), which is one of the most productive basins of Greece in terms of agriculture. To support sustainable water resources management in the PRB, we set up and calibrated the mGROWA hydrological model at a high spatial (100 m) and temporal (daily) resolution for the period 1971–2000, with particular attention given to deriving crop-specific irrigation requirements. We developed and implemented a comprehensive methodological framework to overcome data scarcity constraints in the PRB, thus enabling the derivation of high-resolution spatially continuous estimates of many input variables required for the mGROWA model. We generated estimates of spatiotemporal variations in the water balance components actual evapotranspiration, irrigation requirements, total runoff, and groundwater recharge for the PRB. In addition, through the calculation of indices, such as the potential irrigation to groundwater recharge ratio (PIQR), we demonstrate a way to identify potential unsustainable water use in irrigated agriculture. The established mGROWA model can be used both as a hydrological reference model providing continuous decision support for water resources management, focusing on irrigation water use, and a basis for climate impact studies for the PRB.

Abstract. Human-caused climate change increases the occurrence and severity of droughts due to increasing temperatures, altered circulation patterns and reduced snow occurrence. For example, Europe has suffered from drought events in the last decade like never since the beginning of weather recording. Here we present soil moisture data from 65 Cosmic-ray neutron sensors (CRNS) in Europe (COSMOS-Europe for short) covering recent drought events. The CRNS sites are distributed across Europe and cover all major land use types and climate zones in Europe. The raw neutron count data from the CRNS stations were provided by 23 research institutions and processed using state-of-the-art methods. The harmonised processing included correction of the raw neutron counts, and a harmonised methodology for the conversion into soil moisture based on available in-situ information. In addition, information on the data uncertainty is provided with the dataset, information that is particularly useful for remote sensing and modelling applications. This paper presents the current spatiotemporal coverage of CRNS stations in Europe and describes the protocols for data processing from raw measurements to consistent soil moisture products as well as first results on how the recent drought events have been captured by the CRNS network. This harmonised European soil moisture dataset will help both hydrologists and climate scientists to study individual drought events, to understand their causes, to evaluate and improve their modelling, and to estimate the extremity of current events. The dataset, entitled “Dataset of COSMOS-Europe: A European network of Cosmic-Ray Neutron Soil Moisture Sensors”, is shared via Forschungszentrum Jülich: https://doi.org/10.34731/x9s3-kr48 (Bogena and Ney, 2021).

Zusammenfassung Die Grundwasserneubildung in Festgesteinsbereichen ist aufgrund der hydrogeologischen Eigenschaften der Gesteine im Allgemeinen geringer als in den Porengrundwasserleitern der Lockergesteine. Durch das Zusammenspiel dieser Eigenschaften mit der Topographie der Mittelgebirgslandschaften Niedersachsens, findet dort neben der Grundwasserneubildung auch ein signifikanter Zwischenabfluss statt. Methodisch können in Wasserhaushaltsmodellen im Rahmen einer Abflussseparation diese Größen mithilfe von flächendifferenzierten BFI-Werten berechnet werden. Dabei geben die BFI-Werte das Verhältnis von Grundwasserneubildung zum Gesamtabfluss wieder. Für die Abflussseparation mit dem Modell mGROWA wurden BFI-Werte für das Festgestein von Niedersachsen mit im Vergleich zu vorhergehenden Modellversionen höherem Detaillierungsgrad auf Basis der Geologischen Karte 1:50.000 (GK50) integriert. Dafür wurden 455 geologische Einheiten der GK50 hydrogeologisch analysiert und parametrisiert. Vor allem wasserwirtschaftlich relevante geologische Einheiten geringerer Ausdehnung konnten so besser in der Karte der Grundwasserneubildung berücksichtigt werden. Diesbezüglich werden fünf geologische Einheiten detaillierter diskutiert. Für diese fünf Fallbeispiele wurden mittels eines U‑Tests die Zeitreihen der simulierten jährlichen Grundwasserneubildung auf signifikante Veränderungen geprüft. Es resultiert für alle Zeitreihen eine signifikant veränderte Verteilung der jährlichen Grundwasserneubildung auf einem geringeren Niveau in den gerade vergangenen beiden Dekaden.

Due to their hydrogeological characteristics, groundwater recharge rates in solid rock sites are typically lower than in the porous aquifers of unconsolidated rocks. The interaction of these characteristics with the topography of the low mountain range landscape of Lower Saxony leads to significant interflow in addition to groundwater recharge. In water balance models, these quantities can be calculated using runoff separation procedures by using BFI values. The BFI value specifies the ratio of groundwater recharge to total runoff. To separate runoff with the mGROWA model, we integrated BFI values for the solid rock sites of Lower Saxony with a higher level of detail than in previous model versions through the use of the Geological Map 1:50,000 (GK50). As a result, we obtained an improved representation of geological units with high relevance to water management but small extent in the spatial characterisation of groundwater recharge. For five case study units, we performed a U-test to assess changes and trends in the simulated time series of groundwater recharge. For all case study units we obtained a statistically significant reduced level of the annual groundwater recharge for the past two decades.

To assess climate change impact on the hydrology of Izmit Bay, a coupled model chain using the results of four combinations of Global Climate Models (GCMs) and Regional Climate Models (RCMs) and consisting two hydrological models (mGROWA and PROMET) and one hydrodynamic model (MIKE 3HD) was established. Climate model data of the 4 GCM-RCM combinations were applied to both hydrological models. The resulting 8 streamflow data of the hydrological models were then applied to the MIKE 3HD to assess possible hydrodynamic situations in Izmit Bay. Related model results indicate a range of possible future streamflow regimes suitable for the analysis of climate change impact on Izmit Bay. In order to evaluate the effects of the hydrological changes only on the bay, the bay was considered as closed in terms of hydrodynamics. There is a clear indication that the climate change induced impacts on streamflow may influence the sea level in the Bay to a minor extent. However, climate change induced water exchange processes in the Bay may have a much bigger influence. Hence, it is suggested that further simulations should be run once the hydrologic regime of the Marmara Sea has been assessed in a broader macro-scale study.

In Germany, modelled nitrate concentrations in the leachate are of great importance for the development of scenarios for the long-term achievement of the groundwater quality target according to the specific requirements of the EU Water Framework Directive as well as within the context of the recently adopted general administrative regulation for the designation of nitrate-polluted areas in Germany. For the German federal states of North Rhine-Westphalia (NRW) and Rhineland-Palatinate (RLP), an area-covering modelling of mean long-term nitrate concentrations in leachate with high spatial resolution was carried out using the model system RAUMIS-mGROWA-DENUZ. Hotspot regions with nitrate concentrations in the leachate of 50 mg NO3/L and more were identified for intensively farmed areas in the Münsterland, Lower Rhine, and Vorderpfalz. The validity of modelled values was checked using measured values from 1119 preselected monitoring stations from shallow springs and aquifers filtered near to the surface with oxidizing properties. For the land use categories of urban areas, arable land, grassland, and forest, an at least good agreement of modelled nitrate concentrations in the leachate and measured nitrate concentrations in groundwater was obtained at numerous sites. An equally good agreement was obtained for 1461 measuring stations from the area of responsibility of the Erftverband, which is a major water supplier in the Lower Rhine region. Here, discrepancies have been analyzed in detail due to profound regional knowledge on observation sites. It turned out that in most cases, accuracy limitations of input data (e.g., N balance surpluses of agriculture at the municipal level, 1:50,000 soil map) have been the reason for larger deviations between observed and modelled values. In a broader sense, the case study has shown on the one hand that the model system RAUMIS-mGROWA-DENUZ is able to reliably represent interrelationships and influencing factors that determine simulated nitrate concentrations in the leachate. On the other hand, it has been proven that observed nitrate concentrations in groundwater may provide a solid data source for checking the plausibility of modelled nitrate concentrations in leachate in cases where certain preselection criteria are applied.

In order to analyze the impact of climate change on groundwater resources in North Rhine-Westphalia a multi-model ensemble for projecting future groundwater recharge was established. The ensemble consists of 36 members of the model chain RCP-GCM-RCM-mGROWA in total, i.e. combinations of 3 greenhouse gas concentration trajectories, 6 global and 5 regional climate models, and the water balance model mGROWA. The ensemble projections show only a few significant changes of groundwater recharge in the future periods 2011-2040, 2041-2070, and 2071-2100. A robustness test using the two-criteria model agreement and the significance of the individual model projections did not reveal systematic and significant changes of groundwater recharge until 2100. From the statistical point of view, groundwater recharge can be expected to remain at the current level. Hydrometeorological, North Rhine-Westphalia is located in a transition zone in which the impact of the rising winter precipitation on groundwater recharge is counter-balanced by the impact of warming.

Zusammenfassung Mit einem Multi-Modell-Ensemble wurde analysiert, wie sich der Klimawandel auf den Grundwasserhaushalt in Nordrhein-Westfalen (NRW) auswirkt. Hierzu wurden Projektionen der zukünftigen Grundwasserneubildung für insgesamt 36 Mitglieder der Modellkette RCP-GCM-RCM-mGROWA, bestehend aus 3 RCP-Szenarien zukünftiger globaler Erwärmung, 6 globalen und 5 dynamischen regionalen Klimamodellen sowie dem Wasserhaushaltsmodell mGROWA, vorgenommen. Mit dem Ensemble wurden für die hydrogeologischen Großräume NRWs nur teilweise signifikante Änderungen der jährlichen Grundwasserneubildung in den Perioden 2011–2040, 2041–2070 und 2071–2100 projiziert. Ein Robustheitstest mit zwei Kriterien (Übereinstimmung und Signifikanz der Änderungssignale) liefert keine belastbare Begründung dafür, dass sich die Grundwasserneubildung bis 2100 systematisch und signifikant ändern wird. Aus statistischer Perspektive wird deshalb die Schlussfolgerung gezogen, dass in NRW langfristig eine Grundwasserneubildung erwartet werden kann, die sich nicht grundlegend vom Niveau der Periode 1971–2000 unterscheidet. Hydro-meteorologisch befindet sich NRW in einer Übergangszone, in der eine Zunahme der Winterniederschläge die Wirkung der Erwärmung auf die Grundwasserneubildung wahrscheinlich kompensiert.

Das UBA-Projekt "Auswirkung des Klimawandels auf die Wasserverfügbarkeit − Anpassung an Trockenheit und Dürre in Deutschland", kurz WADKlim, verschafft einen bundesweiten Überblick über die gegenwärtige Wasserverfügbarkeit sowie deren zukünftige Entwicklung unter Klimawandelbedingungen. Im Projekt werden aufkommende Nutzungskonflikte analysiert und mögliche Lösungsstrategien entwickelt. Beispielsweise wird ein Konzept für regionale Wasserbeiräte zur Stärkung intersektoraler Koordination entworfen. Zudem wird das Potenzial von Wasserwiederverwendung zur Bewässerung im urbanen Raum untersucht.

The available renewable water resources of a country are sub-ject to natural interannual variability. However, in Germany, the value of the long-term water availability (of the climate normal period 1961-1990) was exceeded or reached only four times between 2000 and 2018. In particular, the years 2003 and 2018 were characterised by extreme drought and total water supply was significantly below the long-term mean. For the first time since records began, this was followed by a sec-ond significantly dry year, particularly in the east and north of the country. The project "Impact of Climate Change on Water Availability -Adaptation to Drought and Water Scarcity in Ger-many" (WADKlim) funded by the German Environment Agen-cy (UBA) investigates the effects of drought and water scarcity on rivers and lakes as well as on water supply, soil water bal-ance and groundwater availability in Germany. Detailed litera-ture studies and the implementation of mGROWA, TSMP and WaterGAP3 models allow the identification of regional hot-spots with regard to the impacts of water scarcity and drought on water-related sectors and aquatic ecosystems. Further-more, the project analyses current and future water use con-flicts in Germany, and evaluates and develops measures to avoid these conflicts. This includes the development of a con-cept for regional water councils to strengthen coordination between water-related sectors. The project also analyses op-portunities and risks related to water reuse for irrigation in ur-ban areas.

Climate change increases the occurrence and severity of droughts due to increasing temperatures, altered circulation patterns, and reduced snow occurrence. While Europe has suffered from drought events in the last decade unlike ever seen since the beginning of weather recordings, harmonized long-term datasets across the continent are needed to monitor change and support predictions. Here we present soil moisture data from 66 cosmic-ray neutron sensors (CRNSs) in Europe (COSMOS-Europe for short) covering recent drought events. The CRNS sites are distributed across Europe and cover all major land use types and climate zones in Europe. The raw neutron count data from the CRNS stations were provided by 24 research institutions and processed using state-of-the-art methods. The harmonized processing included correction of the raw neutron counts and a harmonized methodology for the conversion into soil moisture based on available in situ information. In addition, the uncertainty estimate is provided with the dataset, information that is particularly useful for remote sensing and modeling applications. This paper presents the current spatiotemporal coverage of CRNS stations in Europe and describes the protocols for data processing from raw measurements to consistent soil moisture products. The data of the presented COSMOS-Europe network open up a manifold of potential applications for environmental research, such as remote sensing data validation, trend analysis, or model assimilation. The dataset could be of particular importance for the analysis of extreme climatic events at the continental scale. Due its timely relevance in the scope of climate change in the recent years, we demonstrate this potential application with a brief analysis on the spatiotemporal soil moisture variability. The dataset, entitled “Dataset of COSMOS-Europe: A European network of Cosmic-Ray Neutron Soil Moisture Sensors”, is shared via Forschungszentrum Jülich: https://doi.org/10.34731/x9s3-kr48 (Bogena and Ney, 2021).

Reaching the EU quality standard for nitrate (50 mg NO3/L) in all groundwater bodies is a challenge in the Federal State of North Rhine-Westfalia (Germany). In the research project GROWA+ NRW 2021 initiated by the Federal States’ Ministry for Environment, Agriculture, Nature and Consumer Protection, amongst other aspects, a model-based analysis of agricultural nitrogen inputs into groundwater and nitrate concentration in the leachate was carried out. For this purpose, the water balance model mGROWA, the agro-economic model RAUMIS, and the reactive N transport model DENUZ were coupled and applied consistently across the whole territory of North Rhine-Westfalia with a spatial resolution of 100 m × 100 m. Besides agricultural N emissions, N emissions from small sewage plants, urban systems, and NOx deposition were also included in the model analysis. The comparisons of the modelled nitrate concentrations in the leachate of different land use influences with observed nitrate concentrations in groundwater were shown to have a good correspondence with regard to the concentration levels across all regions and different land-uses in North Rhine-Westphalia. On the level of ground water bodies (according to EU ground water directive) N emissions exclusively from agriculture led to failure of the good chemical state. This result will support the selection and the adequate dimensioning of regionally adapted agricultural N reduction measures.

Excessive nitrate inputs into groundwater have been recognized as a main reason for failing drinking water standards since decades. Agricultural N-emissions originating from mineral or organic fertilizers are regarded as the most relevant source of nitrate in groundwater worldwide. Accordingly, strategies to cope with the nitrate pollution of groundwater are focused on controlling the agricultural sources of nitrate. In Europe this is reflected in the water legislation on EU level, i.e. the EU Water Framework Directive (EU-WFD), the EU Marine Strategy Framework Directive and the EU Nitrates Directive, obliging the polluter to implement measures to reduce the nitrogen impact on groundwater.With an average population density of 525 inhabitants/km2 the Federal State of North Rhine-Westphalia represents an example for a densely populated region in Germany. Consequently, the assessment of water bodies showed that a number of groundwater and surface water bodies are not in good status due to high nitrogen loads resulting e.g. in high nitrate concentrations in groundwater. There is a debate in North Rhine-Westphalia to what extent agricultural and non-agricultural N-emissions contribute to high nitrate concentrations.The German Working Group on water issues of the Federal States and the Federal Government, require that the nitrate concentration in the leachate should not exceed 50 mg NO3/l. Against this background it is obvious that the nitrate concentration in the leachate represents a decisive parameter for both, the assessment on the nitrate pollution of groundwater and as starting point to determine the N reduction requirements.We used an interdisciplinary model network consisting of a nutrient balance model, a nutrient balancing model (RAUMIS, Henrichsmeyer et al., 1996), a water balance model (mGROWA, Hermann et al., 2015), a reactive nitrate transport model in soil (DENUZ, Wendland et al., 2009) and a reactive nitrate transport model in groundwater (WEKU, Kunkel & Wendland, 1997) to predict the nitrogen intakes and the nitrogen losses to groundwater and surface waters from different input sources and pathways.The nitrogen flux was modelled using nitrogen input data from the time period 2014-2016 and hydrological data for the time period 1981-2010. The nitrate concentrations in the leachate were calculated separately for agricultural and non agricultural N-sources involved, to enable the identification of the main polluter in a certain region, i.e. the one who has to implement measures to to reduce the nitrogen impact on groundwater.From the model analysis it becomes evident that non-agricultural sources do only locally cause nitrate concentrations in the leachate above 50 mg NO3/l in spite of the high population density (525 inhabitants / km2). It could be confirmed that agricultural sources (N-balance surpluses from agriculture and atmospheric NH4 deposition) are exclusively responsible for extended areas of nitrate concentrations above 50 mg NO3/l. Especially in the northern (Münsterland) and western (Lower Rhine basin) parts of the Federal State the implementation of measures to reduce agricultural N-emissions in the context of the WFD program of measures is necessary. These results will not only support the right dimensioning of agricultural N-reduction measures, but also affect the selection and implementation of regionally adapted N-reduction measures.