Water quality assessment and management of lowland river catchment This dissertation describes at first the hydrology and the long term impact of point and diffuse source pollution on nutrient loads based on the current agricultural practices and sewage disposals in rural lowland catchments. The evaluations of Best Management Practices (BMPs) for water quality improvement was then implemented aiming at controlling and reducing pollution from point and diffuse sources in the entire catchments. The study catchments including Kielstau catchment (50 km²) and its subcatchment (7.6 km²) namely Moorau catchment are located in the North German lowlands.