Abstract
Abstract. The aims of this study are to identify the capacities of applying an ecohydrological model for simulating flow and to assess the impact of point and non-point source pollution on nitrate loads in a complex lowland catchment, which has special hydrological characteristics in comparison with those of other catchments. The study area Kielstau catchment has a size of approximately 50 km2 and is located in the North German lowlands. The water quality is not only influenced by the predominating agricultural land use in the catchment as cropland and pasture, but also by six municipal wastewater treatment plants. Ecohydrological models like the SWAT model (Soil and Water Assessment Tool) are useful tools for simulating nutrient loads in river catchments. Diffuse entries from the agriculture resulting from fertilizers as well as punctual entries from the wastewater treatment plants are implemented in the model set-up. The results of this study show good agreement between simulated and measured daily discharges with a Nash-Sutcliffe efficiency and a correlation coefficient of 0.76 and 0.88 for the calibration period (November 1998 to October 2004); 0.75 and 0.92 for the validation period (November 2004 to December 2007). The model efficiency for daily nitrate loads is 0.64 and 0.5 for the calibration period (June 2005 to May 2007) and the validation period (June 2007 to December 2007), respectively. The study revealed that SWAT performed satisfactorily in simulating daily flow and nitrate loads at the lowland catchment in Northern Germany.
Highlights
The degradation of water quality due to non-point source and point source pollution is becoming an increasing global concern
The objective of this paper is to evaluate the performance of the SWAT model in simulating water balance and stream discharge in a complex lowland catchment which has special hydrological characteristics in comparison with those of other catchments, and to predict the impact of point and nonpoint source pollution on nitrate loads based on current agricultural practices and sewage disposals at the watershed outlet
One wastewater treatment plant has been built in Moorau tributary, three others in Hennebach tributary and two others in the Kielstau River; all of which can have a remarkable influence on the water quality of the Kielstau River downstream (Schmalz et al, 2007)
Summary
The degradation of water quality due to non-point source and point source pollution is becoming an increasing global concern. In order to improve the quality of polluted water bodies, the European Framework Directive was implemented in the year 2000 to protect the various types of water bodies in question (EC, 2000). Lowland catchments are ecosystems with low flow velocity, a high ground water table, and flat topography (Muller et al, 2004; Krause et al, 2007; Schmalz et al, 2008). Agricultural practices such as fertilizer and pesticide use as well as sewages are main reasons causing the pollution of stream water in these catchments in Northern Germany. Evans et al (1995) have suggested the implementation of controlled drainage as management practices to minimize nitrate losses. David et al (1997) have found high nitrate concentrations with the range of 5 to 49 mg/l in drainage tiles in an agricultural catchment area in Illinois
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