The concept of water-sensitive cities continues to gain traction globally, as the disruptive effects of urbanisation on local hydrological processes and the potential benefits of green infrastructure become increasingly evident. Despite this, in many planning instances, consideration is only given to how the water balance will be altered and hazard risk reduced from the current urbanised state to the state after implementation of green infrastructure. Why is the understanding of the natural water balance in the pre-urbanisation state often not considered as reference point for planning? If urban green infrastructure should provide hydrological and ecosystem services, should these services be similar to those in the natural condition before urbanisation?For our study, we recreated the daily near-natural water balance for the city of Hamburg to quantify how urbanisation has already affected the water balance, particularly in years of hydrological extremes that represent hydrological hazards. Using the fully-distributed daily water balance model mGROWA, we developed two very high resolution (25 m) models for the city of Hamburg for 1991–2020; one representing the current hydrological situation and one representing a theoretical near-natural situation. To generate the near-natural scenario, the input datasets for topography, soil and land cover were adjusted through the integration of various datasets representing non-anthropogenic conditions, while sealed surfaces and artificially drained areas were removed from the datasets. As expected, due to the lack of runoff from sealed surfaces the actual evapotranspiration is much higher (+40%) in the near-natural scenario than in the current one. Groundwater recharge was also higher in the near-natural scenario (+27%), mainly due to the lack of surface sealing. We then compared the water balance components for the two models against the SPEI meteorological drought index to assess differences in the extremely wet and extremely dry periods that represent potential hydrological hazards. This revealed an increasing divergence in some water balance components between the scenarios for the extreme conditions, quantifying the extent to which the urbanisation of the city has exacerbated hydrological hazard risks. Our study presents a transferable methodology for assessing how urbanisation has affected the natural water balance of a region, which can be used as a starting point for defining targeted solutions for green infrastructure, with the aim of achieving water-sensitive cities.

For the first time, the AGRUM model consortium—consisting of the agro-economic model RAUMIS, the water balance model mGROWA, the hydrological nutrient transport models DENUZ, WeKu and MEPhos, and the urban emission model MONERIS—was jointly set up throughout Germany (357,000 km2). This provided a nationwide consistent nutrient model to capture the current status of N and P inputs to surface waters from diffuse sources and urban areas. Diffuse nutrient emissions were quantified in high spatial resolution for the input pathways’ groundwater, drainage runoff, and natural interflow (100 m × 100 m), as well as for water erosion and wash-off (25 m × 25 m). The sum of diffuse nutrient inputs to surface waters is about 385,000 metric tons N/yr and ca. 11,530 metric tons P/yr. Urban emissions were quantified either as point source inputs (wastewater treatment plants, industrial direct dischargers) or at municipality scale for different collection and treatment systems, e.g., rainwater sewers or decentralized treatment plants, and sum up to ca. 95,000 t N/yr and 7500 t P/yr. As modelled, total N and P inputs into surface waters correspond well with observed N and P loads in rivers. The model results represent valuable information for water managers, being responsible for the preparation of management plans for the third management cycle of the EC Water Framework Directive spanning from 2021 to 2027.

This study presents AGRUM-DE and its results. It is the joint project of agriculture and water management administration for nationwide nutrient modelling in Germany. The Federal/Lander Working Group on Water and the Thunen Institute financed the AGRUM-DE project. In addition to building a nationwide consistent nutrient model, the project aimed to provide the river basin communities with information for the preparation of management plans for the third management cycle (2021 to 2027) of the EC Water Framework Directive. A project advisory board with 51 members from the agricultural and water management administrations of the federal and state governments accompanied the study. There the individual steps of the modelling and the model results were presented, discussed and partly adapted in a comprehensible manner. The direct involvement of the above-mentioned administrative units has helped to establish the broadest possible acceptance of the modelling approach at federal and Land level. The federal government has subsequently decided to continue the AGRUM model network for nationwide modelling as part of the "impact monitoring" required of Germany by the European Commission to review the impact of the Fertiliser Ordinance on groundwater and surface waters and to develop it further together with the Lander.

For the Hessian river basins, an area-differentiated modeling of the nitrogen input to the groundwater and surface waters was carried out for six diffuse input pathways and six point source input pathways on the basis of the geodata available at the state level. In this context, extensive plausibility checks of the model results were carried out using the data from several official monitoring networks at the state level. These include the comparison of modeled runoff components and input pathways for nitrogen using the data from the network of discharge monitoring stations. For the validation of the modeled nitrate concentrations in the leachate, the data from groundwater monitoring wells for controlling the chemical status of groundwater were used. The validation of the modeled nitrate inputs to the groundwater and denitrification in the groundwater was carried out using the data from a special monitoring network of groundwater monitoring wells that include N2/Ar measurements. The data from the Surface Water Quality Monitoring Network were used to verify the plausibility of the modeled total N inputs to the surface waters from diffuse sources and from point sources. All of the model results evaluated by the plausibility checks prove that the nitrate pollution situation in Hesse is adequately represented by the model. This is a prerequisite for accepting the model results at the state level as a basis for developing and implementing regionally appropriate mitigation measures. The Hessian State Agency for Nature Conservation, Environment and Geology uses the model results in the broader context of the work on implementing the EU Water Framework Directive and the EU Nitrate Directive.

The input of nutrients into surface waters and groundwater is directly linked to runoff components. Due to the different physicochemical behaviour of nitrogen and phosphorus compounds, the individual runoff components have different significance as input pathways. Within the scope of the Germany-wide project AGRUM-DE, spatially differentiated runoff components were modelled with the water balance model mGROWA at a resolution of 100 m. The modelled distributed runoff components include total runoff, surface runoff, drainage runoff, natural interflow, direct runoff from urban areas, and groundwater recharge. Although the mGROWA model operates in daily time steps, modelled runoff components can be aggregated to mean long-term hydrologic reference periods—for this study, 1981–2010. We obtained good model agreement through the comparison of measured discharge from 298 river gauges against the spatial means of the modelled runoff components over their corresponding catchment areas. Therefore, the model results provide reliable input for input pathway-specific modelling of actual nutrient inputs as well as scenario analyses expected from the application of nutrient reduction initiatives. This ensures that any differences in the model results stem exclusively from differences in nutrient supply (fertilisation of the soils) and not from climatic effects, such as the influence of wet or dry years.

Background Conversion of leached and runoff nitrate (NO3-) from agricultural land into emissions of the greenhouse gas (GHG) nitrous oxide (N2O) by denitrification in water bodies has to be reported in national GHG inventories. The global IPCC default methodology for estimating these indirect N2O emissions assumes that a fixed fraction of nitrogen (N) inputs (Frac(leach)) is lost through leaching and runoff. However, this method does not consider all relevant country-specific conditions that may influence NO3- leaching. Aims The aim of this study was to apply a model-based approach for estimating indirect N2O emissions through NO3- leaching and runoff from agricultural soils for use in Germany's national GHG inventory. Methods High-resolution spatial data and a comprehensive model system (RAUMIS-mGROWA-DENUZ) were used to derive regionally differentiated and temporarily dynamic Frac(leach) values from N surplus and hydrogeological conditions. These were then used to estimate indirect N2O emissions in accordance with the IPCC methodology. Results The nationwide average of the new implied Frac(leach) values was 0.099 kg N (kg N input)(-1) in 2019. The new estimate of indirect N2O emissions was 10.4 Gg N2O in 1990 and 5.7 Gg N2O in 2019, which are 27 and 52% less than the calculation based on the 2006 IPCC Tier 1 methodology. Conclusions The model-based method for estimating Frac(leach) incorporates relevant factors that influence NO3- leaching and runoff and considers site-specific, spatially varying conditions and differences in the agrarian structure. The use of N surplus as the model driver allows annual changes in cropping conditions and the effects of N-regulating policies and mitigation measures to be represented.

Denitrification in groundwater of aquifers is an important process that helps to maintain environmental standards, yet there is a limited number of studies that determine the spatial variation of denitrification conditions in aquifers on a regional scale. This paper presents a procedure for the regionally differentiated derivation of denitrification conditions in groundwater based on measured values of the redox-sensitive parameters oxygen, iron, manganese, DOC and nitrate as well as information on aquifer typologies. We applied this procedure to Germany, using measured values from more than 24,000 groundwater monitoring sites from 2007-2016. Annual concentrations of the five parameters at the monitoring sites were regionalized using an optimized, iterative inverse distance weighting procedure, using 15 aquifer typologies for spatial delineation. The annual grids (2007-2016) of each parameter were then overlaid and a median over time was calculated. Discrete ranks were then assigned to the concentrations of each parameter depending on their redox class, and ultimately, after overlaying the five parameters, a mean value was calculated describing the nitrate degradation capacity in groundwater.To assess the plausibility of the derived denitrification conditions in groundwater, the latter were linked to reaction constants of denitrification in groundwater that assume a dependence of the extend of denitrification on the travel time in groundwater. A comparison of the modeled quantities of denitrified nitrate with the proportion of denitrified nitrate determined with the N 2 /Ar method at 820 groundwater monitoring wells in three German Federal States showed good agreement. Accordingly, the method presented here is suitable to be used consistently for larger regions for the regionally differentiated derivation of denitrification conditions in groundwater. For regions with denitrifying groundwater conditions, the results provide an explanation for the frequently observed discrepancy between high nitrate emissions from soil and low nitrate concentrations in the groundwater of intensively used agricultural areas.

Denitrification in groundwater is an important process that helps to maintain environmental standards, yet there are very few studies that determine the spatial variation of denitrification conditions in aquifers on a regional scale. We introduce a procedure to derive spatially continuous estimates of denitrification conditions in groundwater based on the interpolation of measurements of the redox-sensitive parameters oxygen, nitrate, iron, manganese and DOC, combined with the quantification of denitrification using a 2D-hydrodynamic model based on first-order reaction kinetics. We applied this procedure to Germany, using measured values from more than 24,000 groundwater monitoring sites from 2007 to 2016. Annual concentrations of the five parameters at the monitoring sites were regionalized using an optimized, iterative inverse distance weighting procedure within 15 aquifer typologies for spatial delineation. The annual grids (2007–2016) of each parameter were then overlaid and a median over time was calculated. Discrete ranks were then assigned to the concentrations of each parameter based on their redox class, and ultimately, after overlaying the five parameters, a mean value was calculated describing the nitrate degradation conditions in groundwater. After assigning half-life times and reaction constants to those denitrification conditions, we quantified denitrification in groundwater using the hydrodynamic model WEKU.To assess the plausibility of the derived denitrification in groundwater, we compared our results with the proportion of denitrified nitrate determined with the N2/Ar method at 820 groundwater monitoring wells in three German Federal States, which showed an overall good agreement. Accordingly, the method presented here is suitable to be used for the regionally differentiated derivation of denitrification conditions in groundwater. For regions with denitrifying groundwater conditions, the results provide an explanation for frequently observed discrepancies between high nitrate emissions from the soil and low nitrate concentrations in the groundwater of intensively used agricultural areas.

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.