Kväveläckage inom Tolångaåns dräneringsområde : modellering och åtgärdssimulering

Sammanfattning: Human-induced eutrophication has rapidly become one of the most important environmental issues. The problem is largely due to the shift in agricultural practices. Farming methods have become more efficient during the last 300 years. Watercourses have been straightened out and wetlands have been reduced to increase agricultural landuse. Since the end of the 1940s, fertilization has increased dramatically. These factors have caused unnatural eutrophication and algae blooming in watercourses, lakes and ultimately the sea. In Sweden, the government has determined to reduce the nitrogen discharge by 50 percent by the year 2000. In order to accomplish this reduction companies and authorities have been assigned to find suitable reduction measures. The soils in Scania are among the most nutritious ones in Sweden. This along with high fertilization makes Scania a high risk area. In 1991, Ekologgruppen was assigned by the government to investigate the drainage area of the stream of Kävlingeån. Tolångaån constitutes a part of Kävlingeån and measurements of the water quality have been taking place since 1976 at Tolånga. As there were plenty of data available in this area the decision was made, in agreement with the county administrative board in Malmö, to study Tolångaån. The fact that the water finally reaches the lake of Vombsjön highlights the importance of establishing a good quality of the water. The lake serves as a source of water supply and provides parts of Scania with drinking-water. The study focuses on investigating the nitrogen situation in the stream and comparing measured values with simulated ones. Nitrogen simulation models are usefool tools, when making predictions of leakage. The model used in this study is based on the SOILN-model and simulates nitrogen dynamics and losses in the agricultural soil of the study area. SOILN was developed in the mid 80s and simulated leakage has shown good accuracy with observed measurements. The simulation was made over a ten-year period (1985-1994). The leakage from four different crops were simulated; ley, seed, rootcrops and oil plant. In addition, the effect of one measure to reduce leakage was simulated, the use of catch cropping. To illustrate the effect of the climate, one year with low precipitation (1989) and one year with high precipitation (1994) were compared. A trend analysis shows that nitrogen content in the watercourse has been fairly constant during the last two decades. However, the nitrogen content is far too high to be acceptable. When comparing measured values with simulated values adjustments had to be made. The simulated leakage was too high and a model parameter was evaluated in order to adjust the leakage. In spite of this, simulated values did not agree totally with measured values. This is probably due to the fact that the ground water model which is connected to the nitrogen model is too simple. When comparing the leakage caused by different crops, it turned out that ley had the best nitrogen uptake. Seed and root-crops were not as efficient as ley when it comes to assimlate nitrogen and contributed to most of the leakage. When simulating the effect of catchcrops it was possible to reduce nitrogen leakage by 30 percent. Previous studies have shown that catchcrop may reduce nitrogen leakage by 60 percent. The results from Tolånga and the simulation showed that precipitation is the parameter that has the greatest influence on the nitrogen leakage.