GIS-based Sediment and Nutrient Balancing for Mesoscaled Watersheds: the STOFFBILANZ Approach
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ISSN der Zeitschrift
Informatics for Environmental Protection - Networking Environmental Information
Geographical information systems and their applications
Masaryk University Brno
The identification of non-point sources in catchment areas and the quantification of the resulting phosphorus and nitrogen loads into the watercourses are necessary to take measures of cultivation management and of control, in order to preserve and protect soil and water quality standards especially regarding the EU Water Framework Directive. The following paper presents a modeling approach, mainly based on empirical studies and advanced GIStechnologies. Due to the very complex system, the interactions between the factors of soil, water, vegetation, geology, climate and human impacts have to be considered in a sufficient way. Indeed the modeling in mesoscaled river catchment areas needs some simplifying which corresponds to the resolution of the input parameters. The same applies to the management of transfers and processes, which must be derived by available indicators if a numeric calculation is not possible. In order to facilitate the quantifications the procedures of STOFFBILANZ consists of different modules which are acting jointly. The transferability to mid-European landscapes has been tested successfully in several catchments areas in Germany. A core of the model is the consideration, quantification and evaluation of hydrologic connectivity by surface runoff in large watershed areas. Hydrologic connectivity explains the runoff-based interactions between model units or simply between areas and can be defined as these relationships between the components of the water cycle leading to the transfer of matter, energy and genes. The difficulties in implementing such spatially located and temporally variable relations in mesoscaled model approaches are obvious because of their complex nature. Natural as well as anthropogenic factors affect the hydrological connectivity. The problem of scale, coupled with down- / upscaling limitations, scale-specific characteristics of relevant parameters, emergence of new scale-specific phenomena make the development of appropriate approaches both from technical view and specialist knowledge difficult. Based on a probabilistic approach, the procedures implemented in STOFFBILANZ enable hydrologic connectivity in two ways: on the one hand the pure connectivity between areas and waters by surface runoff is considered (quantitative aspect), on the other hand a probability of connectivity is calculated by probabilistic techniques in order to express the capability of any area to contribute to the sediment load of waters (qualitative aspects). Due to that procedure, the model is able to differentiate spatially between areas (1) without any connectivity to rivers by surface runoff, (2) with partial or subordinated connectivity and (3) with high hydrologic connectivity inside of large watershed areas. Subordinated hydrologic connectivity means a hydrologic connection without relevance for sediment inputs to waters. High hydrologic connectivity indicates areas with a high potential for sediment delivery associated to the corresponding particulate bound nutrients. Areas with a high hydrologic connectivity have a part of ten percent or less of a watershed area, but they contribute app. 75 percent to the river load with sediments and particulate bound nutrients. Thus, the identification of spatially distributed source areas is a major challenge for mesoscale modeling.