Sensitivity of mGROWA-simulated groundwater recharge to changes in soil and land use parameters in a Mediterranean environment and conclusions in view of ensemble-based climate impact simulations.

This study examines the impact of changing climatic conditions on groundwater recharge in the Riu Mannu catchment in southern Sardinia. Based on an ensemble of four downscaled and bias corrected combinations of Global and Regional Climate Models (GCM-RCMs), the deterministic distributed water balance model mGROWA was used to simulate long-term mean annual groundwater recharge in the catchment for four 30-year periods between 1981 and 2100. The four employed GCM-RCM combinations project an adverse climatic development for the study area: by the period 2071-2100, annual rainfall will decrease considerably, while grass reference evapotranspiration will rise. Accordingly, ensemble results for our base scenario showed a climate-induced decrease in the median of annual groundwater recharge in areas covered by Macchia from 42-48mm/a to 25-35mm/a between the periods 1981-2010 and 2071-2100, corresponding to a reduction of 17-43%. To take into account the influence of additional plant available water storage in weathered bedrock on groundwater recharge generation, the model was extended by a regolith zone for regions covered by Mediterranean Macchia. In a set of model runs ("scenarios"), parameter values controlling the water storage capacity of this zone were increased step-wise and evaluated by comparison to the base scenario to analyze the sensitivity of the model outcome to these changes. The implementation of a regolith zone had a considerable impact on groundwater recharge and resulted in a decrease of the median in annual groundwater recharge: by 2071-2100, the 35% scenario (available water content in the regolith of 3.9 to 5.7vol.%) showed a reduction of 67-82% as compared to the period 1981-2010 in the base scenario. In addition, we also examined the influence of changes in the crop coefficients (Kc) as well as different soil texture distributions on simulated groundwater recharge.

P-pollution in a heavily urbanized river basin from point and diffuse sources: the River Ruhr case study (Germany).

An area-differentiated model approach (MEPhos) for the quantification of mean annual P-inputs from point and diffuse sources is presented. The following pathways are considered: artificial drainage, wash-off, groundwater outflow, soil erosion, rainwater sewers, combined sewer overflows, municipal waste water treatment plants and industrial effluents. Based on the modelling results "hot spots" for high P-loads can be localized and management option for the input reduction to surface waters proposed. The model is applied to the River Ruhr basin (4,485 km2) in Germany with contrasting natural conditions, land use patterns as well as population and industry densities.

The influence of diffuse pollution on groundwater content patterns for the groundwater bodies of Germany.

Commissioned by Germany's Working Group of the Federal States on Water Problems (LAWA) the authors developed a procedure to define natural groundwater conditions from groundwater monitoring data. The distribution pattern of a specific groundwater parameter observed by a number of groundwater monitoring stations within a petrographically comparable groundwater typology is reproduced by two statistical distribution functions, representing the "natural" and "influenced" component. The range of natural groundwater concentrations is characterized by confidence intervals of the distribution function of the natural component. The applicability of the approach was established for 17 hydrochemical different groundwater typologies occurring throughout Germany. Based on groundwater monitoring data from ca. 26,000 groundwater-monitoring stations, 40 different hydrochemical parameters were evaluated for each groundwater typology. For all investigated parameters the range of natural groundwater concentrations has been identified. According to the requirements of the EC Water Framework Directive (article 17) (WFD) this study is a basis for the German position to propose criteria for assessing a reference state for a "good groundwater chemical status".

Area-differentiated modelling of P-fluxes in heterogeneous macroscale river basins.

A model (MEPhos) for the calculation of the total phosphorus output from diffuse sources by drainage, wash-off, groundwater outflow, soil erosion and rainwater sewers as well as from point sources is presented. The model is based on a pathway- and area-differentiated emissions approach and calculates mean long-term P-inputs to surface waters. Phosphotopes are used for spatial discretization and modelling of diffuse P-inputs. Based on the modelling results "hot spots" for high P-loads can be localized and a management option for the input reduction to surface waters can be proposed which are adapted to site properties. The applicability of the model is demonstrated for two macroscale river basins in Germany (- 13,000 km(2) each) with contrasting natural conditions and land use patterns.

Water fluxes and diffuse nitrate pollution at river basin scale: coupling of agro-economic models and hydrological approaches.

An integrated model system has been developed to estimate the impact of nitrogen reduction measures on the nitrogen load in groundwater and in river catchment areas. The focus lies on an area-wide, regionally differentiated, consistent link-up between the indicator "nitrogen balance surplus" and nitrogen charges into surface waters. As a starting point of the analysis actual nitrogen surpluses in the soil were quantified using the agro-economic RAUMIS-model, which considers the most important N-inputs to the soil and N-removals from the soil through crop harvest. The most important pathways for diffuse nitrogen inputs into river systems are modelled with the water balance model GROWA. Additionally, the time-dependent nitrogen degradation along the nitrogen pathways in soil and groundwater are modelled using the WEKU-model. The two selected river basins in Germany cover a variety of landscape units with different hydrological, hydrogeological and socio-economic characteristics. The results indicate a wide range of annual nitrogen surpluses for the rural areas between than 10 kg N ha(-1) x a(-1) and 200 kg N ha(-1) x a(-1) or more, depending on the type and intensity of farming. The level of nitrogen inputs into the surface waters is reduced because of degradation processes during transport in soil and groundwater. Policy impact analyses for a nitrogen tax and a limitation of the livestock density stress the importance of regionally adjusted measures.

A procedure to define natural groundwater conditions of groundwater bodies in Germany.

Commissioned by Germany's Working Group of the Federal States on Water Problems (LAWA) the authors developed a procedure to define natural groundwater conditions from groundwater monitoring data. The distribution pattern of a specific groundwater parameter observed by a number of groundwater monitoring stations within a petrographically comparable groundwater typology is reproduced by two statistical distribution functions, representing the "natural" and "influenced" components. The range of natural groundwater concentrations is characterized by confidence intervals of the distribution function of the natural component. The applicability of the approach was established for four hydrochemically different groundwater typologies occurring throughout Germany. Based on groundwater monitoring data from 7920 groundwater monitoring stations, 15 different hydrochemical parameters were evaluated for each groundwater typology. For all investigated parameters the range of natural groundwater concentrations has been identified. According to the requirements of the EC Water Framework Directive (article 17) (WFD) this study is a basis for the German position to propose criteria for assessing a reference state for a "good groundwater chemical status".

Management of regional German river catchments (REGFLUD) impact of nitrogen reduction measures on the nitrogen load in the River Ems and the River Rhine.

The REGFLUD-project, commissioned by Germany's Federal Research Ministry (BMBF), addresses the problem of reducing diffuse pollution from agricultural production. The objective of the project is the development and application of multi-criteria scientific methods, which are able to predict diffuse pollution in river basins subject to economic feasibility and social acceptability. The selected river basins (Ems and Rhine basins) cover a variety of landscape units with different hydrological, hydrogeological and socio-economic characteristics. This paper focuses on the analysis of the effects of certain policy measures to reduce diffuse pollution by nitrogen. For this purpose a model system consisting of an agricultural sector model, a water balance model and a residence time/denitrification model was developed and applied. First results indicate a wide range of annual nitrogen surpluses for the rural areas between less than 10 kg N/ha up to 200 kg N/ha or more depending on the type and intensity of farming. Compared to the level of nitrogen surpluses the level of nitrogen inputs into the surface waters is relatively moderate because of degradation processes during transport in soil and groundwater. Policy impact analysis for a nitrogen tax and a limitation of the livestock density stress the importance of regionally tailored measures.

Groundwater-borne nitrate intakes into surface waters in Germany.

The nitrogen loads entering the surface waters in Germany via the groundwater path were quantified. For this purpose, the results of a nitrogen balance model (Bach et al., 2000), which considers the most important N-inputs to the soil (manure, inorganic fertiliser, atmospheric deposition) and N-removals from the soil through crop harvest, were combined with the groundwater residence time/denitrification model WEKU (Kunkel and Wendland, 1997; Wendland et al., 2001). The modelled groundwater-borne nitrogen inputs into surface waters were validated using results from the MONERIS model (Behrendt et al., 2000) concerning riverine nitrogen retention, nitrogen inputs from point sources as well as nitrogen inputs through direct run-off (drainage etc.). In the vicinity of surface waters and in solid rock areas, the groundwater borne nitrogen inputs into surface waters are considerably high compared to the inputs into the aquifer due to predominantly unfavourable de-nitrification conditions and short residence times of groundwater. In the North German lowlands, however, the groundwater-borne nitrate inputs into surface waters are considerably low compared to the inputs into the aquifer. There, the residence time of groundwater in the aquifer is high and the groundwater is predominantly oxygen free and contains pyrite and/or organic carbon compounds, allowing a halving of the nitrate loads in the groundwater within a period of 1 to 4 years (see Wendland and Kunkel, 1999).

The SOIL-N/WEKU model system--a GIS-supported tool for the assessment and management of diffuse nitrogen leaching at the scale of river basins.

The SOIL-N/WEKU model system was developed to estimate groundwater-borne nitrogen inputs into river systems. The core of this model system is composed of a soil nitrogen leaching model (SOIL-N) and a groundwater residence time/denitrification model (WEKU). The application of the model system was carried out in the framework of the EU-project RANR (Regional analysis of subsurface nitrogen retention and its impact on the nitrogen export from land to sea) for a macroscale study river basin in Germany (the Uecker basin, ca. 2,400 km2) and a mesoscale study catchment area in Denmark (the Gjern basin, ca. 200 km2). For both catchment areas, the modelled average nitrogen loads leached into the groundwater were about 40 kg N/ha a, while the remaining groundwater-borne nitrogen intake to rivers was quantified to an average of about 2 kg/ha a. The comparison with observed groundwater-borne riverine nitrogen loads showed a very good agreement, proving the key role nitrogen retention in groundwater plays in the two catchment areas.

Model system for the management of nitrogen leaching at the scale of river basins and regions.

In the framework of the EU-project RANR (Regional analysis of subsurface nitrogen retention and its impact on the nitrogen export from land to sea) a model system was developed to estimate groundwaterborne nitrogen inputs into river systems. The core of this model system is composed of a soil nitrogen leaching model (SOIL-N) and a groundwater residence time/denitrification model (WEKU). The application of the model system was carried out for the study catchment areas of the Uecker basin (ca. 2400 km2, Germany) and the Gjern basin (ca. 200 km2, Denmark). For both catchment areas, the modelled average nitrogen loads leached into the groundwater were about 40 kg N/ha a, while the remaining groundwaterborne nitrogen intake to rivers was quantified to an average of about 2 kg/ha a. The comparision with observed groundwaterborne riverine nitrogen loads showed a very good agreement, proofing the key role nitrogen retention in groundwater plays in the two catchment areas. With regard to the generalisation and transfer of the SOIL-N/WEKU model concept we assume that the model can be applicated in catchment areas in the European Pleistocene Lowland, which ranges from the Netherlands in the west to the Baltic States and the Ukraine in the east.