Frank Herrmann

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).

A substantial body of knowledge concerning the functioning of catchments has been derived from the 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).

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.

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.

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.

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.