Frank Wendland

The GROWA98 model for water balance analysis in large river basins—the river Elbe case study

Abstract An area differentiated water balance analysis in the river Elbe basin (German part) for the hydrological period 1961–1990 was carried out using the GROWA98 model. The mean long-term total runoff was modelled as a function of the regional interaction of the site conditions climate, soil, geology, topography and land use. The total runoff was separated into the direct runoff (interflow and surface runoff) and groundwater runoff (base flow) using base-flow indices, depending on area characteristics (e.g. geology, depth for groundwater). In this way, the regional dominant runoff components, i.e. pathways for the displacement of nutrients from soil to surface water were identified. The model results were validated by comparing the calculated runoff levels with measured data from 120 sub-basins of the river Elbe-catchment area. In general, the differences between modelled and measured runoff values were less than 15%, indicating the reliability of the chosen procedure. This allows further model applications, e.g. for the quantification of the diffuse nutrient input into groundwater and surface waters differentiated to the runoff components.

Model based impact analysis of policy options aiming at reducing diffuse pollution by agriculture—a case study for the river Ems and a sub-catchment of the Rhine

Abstract In this paper an integration of the agricultural economic model RAUMIS with the hydrological models GROWA98 and WEKU is presented. The focus lies on an area wide, regionally differentiated, consistent link-up between the indicator “nitrogen balance surplus” and nitrogen charges into surface waters. The model network is used to analyze the status quo situation in the year 1999 for two river catchments in Germany that feature very distinct natural and socio-economic conditions. Regarding agriculture, the study areas include regions with specializations in cash crops, in intensive livestock featuring high nitrogen surplus, and extensive livestock production on permanent grassland. Due to regionally varying hydrological conditions quite different shares of agricultural nitrogen surpluses ranging from 25 to 92% enter surface waters. Furthermore, impacts of alternative nitrogen reduction measures namely a limitation of livestock density and a tax on mineral nitrogen are quantified. Measures of the nitrogen reduction potential and costs in terms of agricultural income forgone are taken into account in the assessment. Results regarding the effects of restricting the livestock density or tax mineral nitrogen highlight that the mitigation of diffuse water pollution problems requires regionally tailored measures.

The influence of nitrate reduction strategies on the temporal development of the nitrate pollution of soil and groundwater throughout Germany - a regionally differentiated case study

A GIS-based area-differentiating model has been used to analyze the nitrate pollution of soil and groundwater throughout Germany. The results of the calculations based on the model for the current situation show that a high potential for high nitrate pollution of the soil and groundwater (> 50 mg NO3/l) is to be expected in all regions of Germany subject to intensive agricultural use. In order to achieve a sustainable use of water resources, effective strategies to reduce the nitrogen surpluses from agriculture must be developed and analyzed with respect to their spatial and temporal impact on the nitrate pollution of soil and groundwater, taking into consideration the various agricultural land usages as well as the different hydrological, hydrogeological and agricultural conditions.The effects of three different nitrate reduction strategies on the resulting N-surpluses and the nitrate concentration in the leachate were investigated: firstly, a stocking rate limitation, secondly, a limitation of both organic and mineral fertilizers and thirdly, a combination of three reduction measures consisting of a stocking rate limitation, an improvement of the nitrogen utilization factor by livestock and a higher utilization factor of nitrogen bound in organic fertilizers by crops. The analysis showed that separate application of each of these nitrogen reduction measures would only lower the nitrogen surpluses in a few regions. In order to achieve a considerable reduction of nitrate concentrations both in leachate from land under agricultural use and in the groundwater a combination of area-covering and regionally effective measures (scenario III) turned out to be most promising.

Modelling the nitrate flow in the ground-water provinces of the “old” federal states of the Federal Republic of Germany

Abstract The aim of this study is to trace the nitrate flow in the ground water from diffuse sources over the whole area of the former West Germany ranging from input into the soil up to output into surface waters. A systems analysis approach was selected for the implementation of these tasks since this permits an interconnection of large volumes of data with complex and interdisciplinary correlations. The problem-relevant aspects were compiled from the profusion of data and regularities on nitrate discharge from the soil, the hydrological and geological circumstances as well as nitrate conversion processes in the soil and ground water, and pieced together to form a comprehensive model. This model was transformed into a computer program and coupled to a graphical program system permitting a clear illustration in the form of coloured grid maps. A high nitrate pollution of the ground water (> 50 mg NO 3 /l) can be expected in all regions with an intensive agricultural use of the topsoil. In particular ground-water occurrences in solid rock areas are susceptible to nitrate pollution. There a rapid ground-water turnover and thus a short residence time for the ground water in the aquifer is typical. Oxidizing aquifer conditions usually prevail in solid rock aquifers preventing nitrate degradation. In many loose rock areas, in contrast, the ground water has a low flow velocity and a long residence time in the aquifer. Because of a lack of free oxygen a complete degradation of nitrate can occur, as long as iron-sulphide compounds and/or organic carbon are available in the aquifer.

Potential nitrate pollution of groundwater in Germany: A supraregional differentiated model

Implemented on behalf of the Federal Ministry for Research and Technology (BMFT), a model is developed to trace the nutrient flow of nitrate in the soil and the groundwater on a supraregional scale. Research work is intended to indicate regionally differentiated hazardous potentials and thereby provide a basis for recommending comprehensive measures to protect groundwater in Germany. The adaption of the model to the hydrogeological and agricultural conditions of other states is possible in principle. This article focuses on the hydrogeological model parts. A high nitrate pollution of groundwater can be expected in all regions with intensive agricultural use of the topsoil. In particular, groundwater in solid rock areas is susceptible to nitrate pollution. There a rapid groundwater turnover and thus a short residence time for the groundwater in the aquifer is typical. Oxidizing aquifer conditions usually prevail in solid rock aquifers, preventing nitrate degradation. In many loose rock areas, in contrast, the groundwater has a low flow velocity and a long residence time in the aquifer. Because of a lack of free oxygen, a complete degradation of nitrate can occur, as long as iron sulfide compounds and/or organic carbon are available in the aquifer. A more detailed presentation of the whole research work is given in Wendland et al. (1993).

Integrated Agricultural and Hydrological Modeling within an Intensive Livestock Region

An interdisciplinary model network consisting of the regional agricultural economic model RAUMIS and the hydro(geo)logical models GROWA/WEKU is used to analyze the effect of different scenarios of maximum agricultural nitrogen balance surplus on water quality. The study area is the federal state of Lower Saxony, Germany, which features heterogeneous natural site conditions as well as agricultural production structures. A focus of the study is the modeling of supra-regional manure transports that, according to the models results, considerably increase due to a lowering of maximum nitrogen balance surpluses. The assessment of the examined nitrogen reduction measures reveals that adequate indicators have to be applied. In this regard, the model results show that even though the analyzed measure leads to a substantial overall reduction of agricultural nitrogen surpluses, nitrogen discharges into surface and groundwater can regionally increase.

Forecasting the effects of EU policy measures on the nitrate pollution of groundwater and surface waters

We used the interdisciplinary model network AGRUM [corrected] to predict the actual mean nitrate concentration in percolation water at the scale of the Weser river basin (Germany) using an area differentiated (100 m x 100 m) approach. AGRUM [corrected] combines the agro-economic model RAUMIS for estimating nitrogen surpluses and the hydrological models GROWA/DENUZ for assessing the nitrate leaching from the soil. For areas showing predicted nitrate concentrations in percolation water above the European Union (EU) groundwater quality standard of 50 mg NO3-N/L, effective agri-environmental reduction measures need to be derived and implemented to improve groundwater and surface water quality by 2015. The effects of already implemented agricultural policy are quantified by a baseline scenario projecting the N-surpluses from agricultural sector to 2015. The AGRUM [corrected] model is used to estimate the effects of this scenario concerning groundwater and surface water pollution by nitrate. From the results of the model analysis the needs for additional measures can be derived in terms of required additional N-surplus reduction and in terms of regional prioritization of measures. Research work will therefore directly support the implementation of the Water Framework Directive of the European Union in the Weser basin.

Simulation of future groundwater recharge using a climate model ensemble and SAR-image based soil parameter distributions — A case study in an intensively-used Mediterranean catchment

We used observed climate data, an ensemble of four GCM-RCM combinations (global and regional climate models) and the water balance model mGROWA to estimate present and future groundwater recharge for the intensively-used Thau lagoon catchment in southern France. In addition to a highly resolved soil map, soil moisture distributions obtained from SAR-images (Synthetic Aperture Radar) were used to derive the spatial distribution of soil parameters covering the full simulation domain. Doing so helped us to assess the impact of different soil parameter sources on the modelled groundwater recharge levels. Groundwater recharge was simulated in monthly time steps using the ensemble approach and analysed in its spatial and temporal variability. The soil parameters originating from both sources led to very similar groundwater recharge rates, proving that soil parameters derived from SAR images may replace traditionally used soil maps in regions where soil maps are sparse or missing. Additionally, we showed that the variance in different GCM-RCMs influences the projected magnitude of future groundwater recharge change significantly more than the variance in the soil parameter distributions derived from the two different sources. For the period between 1950 and 2100, climate change impacts based on the climate model ensemble indicated that overall groundwater recharge will possibly show a low to moderate decrease in the Thau catchment. However, as no clear trend resulted from the ensemble simulations, reliable recommendations for adapting the regional groundwater management to changed available groundwater volumes could not be derived.

Projected impact of climate change on irrigation needs and groundwater resources in the metropolitan area of Hamburg (Germany)

Irrigated agriculture is an important economic factor in the rural parts of the metropolitan area of Hamburg. It is commonly expected that climate change will reduce the groundwater quantities available for field irrigation. Against this background, the ratio of irrigation need and groundwater recharge (IGR-ratio) is suggested as an indicator to assess climate change impacts on the vulnerability of groundwater resources towards overexploitation by agricultural irrigation. The IGR-ratio has been assessed based on the distributed water balance model mGROWA, i.e. under consideration of the simulated groundwater recharge levels and the field crop-specific irrigation need of the commonly cultivated field crops. The spatial IGR-ratio distribution determined for the observed reference period 1971–2000 has shown that the delineated vulnerable areas coincide with the regions for which high irrigation quantities have been documented at present. Additionally, the IGR-ratio depicts the areas in which irrigation is currently still negligible, but in which the introduction of irrigation into agricultural practice would lead to an immediate overexploitation of the sustainably available groundwater budget. The possible impact of future climate on IGR-ratios was determined by using a model chain of mGROWA and the regional climate models REMO and WETTREG2010. The related ensemble simulations did not provide a uniform tendency of possible future IGR-ratio changes. Whereas the mGROWA–WETTREG2010 realisations projected a very high increase in the IGR-ratios, the mGROWA–REMO realisations did not show a pronounced trend of increasing IGR-ratios. Therefore, considerable uncertainties remain regarding the future bandwidth of IGR-ratio changes.

Sustainable use of water resources in Europe and the role of integrated modelling of phosphate fluxes.

Sustainable management of European water resources is advanced by the implementation of the Water Framework Directive. To achieve its aims the directive requires the realisation of cost-efficient programmes of measures. In this context, also measures for reducing phosphate inputs have to be taken, which demands models as quantification tools for the status quo analysis as well as for scenario analyses. An area-differentiated model approach (MEPhos) for the systematic quantification of P-inputs from point and diffuse sources via eight different pathways is presented. Based on the modelling results, source areas of high P-emissions can be localised and management options proposed.