Im Rahmen des Projekts "Hydrologische Güte" wurde ein Verfahren zur Bewertung des hydrologischen Zustands von Flusseinzugsgebieten erarbeitet. Der hier vorgestellte Teilbereich "Stoffhaushalt und Stoffdynamik" hat die Bewertung der Emissionen von Stickstoff und Phosphor sowie der damit verbundenen Belastung der Gewässer (Immission) zum Ziel. In der Studie werden der theoretische Hintergrund, die Entwicklung sowie die Anwendung des Verfahrens in verschiedenen mesoskaligen Flusseinzugsgebieten Baden-Württembergs behandelt.
Die aus unterschiedlichen Quellen stammenden Emissionen werden mittels einer räumlich differenzierten Wasser- und Stoffbilanzierung quantifiziert und anhand von Szenarienberechnungen des "Potentiell Natürlichen Zustands" bewertet. Als Grundlage zur Bewertung der Immission werden gemessene oder simulierte Konzentrationen der Fließgewässer herangezogen und mit Hilfe der aktuellen Umweltqualitätsstandards bewertet. Basierend auf den in der Bewertung festgestellten Defiziten können Maßnahmen zur Verbesserung der Wasserqualität formuliert werden. Zur Prognose der Wirksamkeit von Maßnahmen wurde beispielhaft für ein Testgebiet ein Modellansatz zur zeitlich und räumlich differenzierten Simulation des Stickstofftransports entwickelt und erfolgreich angewendet. Für die verwendeten Modellansätze wurden Sensitivitätsanalysen durchgeführt und darauf basierend die bei der Bewertung zu erwartenden Unsicherheitsbereiche abgeschätzt.
To overcome the lack of catchment oriented assessment procedures in the field of water protection an assessment procedure for the hydrological quality of meso-scale catchments was developed at the Institute of Hydrology, University of Freiburg. The main aim of the project was the development and presentation of an adequate methodology and its application in a set of test catchments. The procedure is divided into the assessment units catchment properties, runoff dynamics and nutrient budgets, water quality and solute dynamics. In this study theoretical backround, development and application of the assessment unit nutrient budget, water quality and solute dynamics are presented.
First an overview on the actual research status concerning water quality and on existing methodologies for the description and assessment of nutrient transport and water quality is given. The methodology of the assessment procedure was developed in selected test catchments and subsequently applied in different river basins in South-West Germany which cover a large variety of scale, natural properties, land use structures and water resources issues. To establish a basis for the application of the procedure a GIS-based catchment analysis was carried out for the investigated river basins. A short description of the assessment of catchment properties and runoff dynamics is given in addition to demonstrate the concept of the entire procedure.
In the framework of the development experimental investigations in sub-catchments of the Dreisam river basin and a study of the capability of a water quality model was carried out. The results of the experimental investigations demonstrate a different temporal behaviour of solutes due to their chemical properties and dominating transport paths. In the modelling study the limits of complex conceptual models in larger areas are revealed.
The methodology of the assessment unit nutrient budget, water quality and solute dynamics is based on established conceptual approaches and uses only readily available input data. In the approach the nutrients nitrogen and phosphorus which are crucial for the problem of eutrophication are addressed.
In the parameter group nutrient budget an assessment of nutrient emissions into surface water and groundwater on the basis of a spatially distributed water and nutrient balance is carried out. The nutrient input is derived from the nutrient surplus which is calculated for all permeable land surfaces based on the land use classification and agricultural statistics using calculation methods and values described in literature.
In the distributed water balance interpolated data of mean annual precipitation and the results of a simulation of the actual evapotranspiration are used. The resulting spatially distributed mean annual discharge is further divided into the runoff components “fast runoff”, “delayed runoff”, and “base flow”. The component “fast runoff” is an estimation of surface runoff, macropore flow and drainage runoff. It is quantified by using empirical functions and spatially distributed data of topography, soil characteristics and land use. Quantification of the base flow component is derived using results from base flow separations in sample catchments which were regionalized for all the investigated river basins. Delayed runoff is quantified from the residue of the two other components. The mobilization of nutrients is estimated using the described runoff components, soil characteristics (field capacity) and a coefficient describing the chemical behaviour. To quantify the transport of sediment associated phosphorus a simplified sediment delivery calculation is executed based on data of topography and land use. Nutrient losses in the vadose zone and groundwater aquifer are estimated based on measured values of groundwater concentrations. The nutrient delivery from point sources is quantified based on statistical data for sewage plants and canalisation as well as data for precipitation and the area of impermeable land surfaces. Nutrient retention and denitrification in the river system is quantified empirically based on the hydraulic impact. To achieve a validation of the nutrient balance the resulting concentrations in surface water are compared to long term means of measured concentrations.
The nutrient budget is assessed referring to the “potential natural status” for which a natural vegetation and the absence of direct human impacts is assumed. For the simulation of the “potential natural status” of nutrient emissions all land use classes are parameterized using the nutrient input and water balance terms for forest vegetation. Finally the nitrogen and phosphorus budgets are assessed by comparing the actual and potential natural status. In an uncertainty analysis the range of potential simulation errors in water and nutrient balance is estimated. The resulting uncertainty range in the assessment can be estimated to a maximum error of one quality class.
In the parameter group water quality and solute dynamics the status of water quality in the surface waters is assessed. The water quality in the river system is quantified based on measured or simulated concentrations of nitrogen and phosphorus and assessed using chemical water quality standards. For catchments with long term measurements the temporal variability is taken into account by an assessment of solute dynamics. For this purpose monthly nutrient loads are calculated and assessed using monthly reference loads. The reference loads are derived from combining monthly values of stream flow with the concentrations levels given in the water quality standard. By the use of stream flow related loads a direct comparison with the nutrient emissions calculated in the nutrient balance is enabled.
From the water and nutrient balance emissions of nitrogen and phosphorus for the actual and the potential natural status are derived in each river basin. The results are classified into quality classes. The calculated nutrient loads are mainly a reflection of land use structures in the catchment, nutrient surplus on the farm lands and the number and size of sewage systems. In the validation of the resulting simulated concentrations in surface waters using the means of long term measurements a satisfying agreement was achieved (Nitrogen: mean relative deviance:-6 %; r² = 0,94; Soluble phosphorus: mean relative deviance: 14 %; r² = 0,88). In the parameter group nutrient budget the quality class is mainly determined by land use and nutrient input in the catchment. The parameter group water quality and solute dynamics is also influenced by the hydrological properties in the catchment (e.g. specific discharge, water storage capacities). In this parameter group a reasonable estimation of the status of water quality in surface waters which is similar to the classification using water quality standards is produced.
In a modelling study using a complex conceptional model for the simulation of nitrogen transport in a sub-catchment of the Dreisam basin problems of data requirement, parameter identification and practical handling occurred. These problems were leading to the conclusion that models of this kind are not appropriate for operational application in the meso-scale. For this reason a model concept for nitrogen transport which is suitable for meso-scale applications is developed in the river basin of the Seefelder Aach. The model is built upon an existing water balance model using its simulation results as input variables. Due to a more simple conceptional approach, problems of data requirement are avoided and parameter uncertainty is reduced. As this model is used for scenario simulations of nutrient management options an assessment of model uncertainties and the resulting potential simulation errors is carried out.
The assessment of nutrient budget, water quality and solute dynamics is carried out in the framework of the application of assessment procedure „hydrological quality”. With the proposed methodology the basis for an spatially orientated assessment of nutrient budgets and water quality in meso-scale catchments is established. Apart from the quantification of the human impact on water quality described in this study human impacts on the water balance and the variability of stream flow are parameterized in the two remaining assessment units catchment properties and runoff dynamics. Therefore the methods compiled in the assessment procedure can be helpful tools for the estimation of the “significant human impacts” on the aquatic environment which has to be included in the river basin management plans according to the new European water framework directive. When applied
along with morphological and biological assessment procedures the assessment of the „hydrological quality” will enable a holistic approach regarding water protection, in which the catchment and its water bodies are treated as a hydrological unit.