This paper focuses on the influences of different data sources, and the variation in spatial resolution of input data
and analysis, on the calculated nutrient emissions using the conceptual model MONERIS. MONERIS calculates both
nitrogen and phosphorus emissions from point and diffuse sources and the riverine nutrient retention. By subtracting
the retention from the emissions, a riverine nutrient load was estimated and compared with the observed nutrient river
load. All calculations were conducted for the period from 1991 to 1993.
The River St¨or, with a catchment area of 1135 km2, located in a postglacial lowland landscape in northern Germany,
was chosen as a case study area. Two different data sets (e.g. land use, soil type or wastewater treatment plant inventory)
were used: a commonly available standard data set (German or European maps) and a more detailed set with a higher
spatial resolution derived from several studies at the Ecosystem Research Centre in Kiel. Initially, both data sets were
used to apply MONERIS to the total catchment. The results were compared to adapt some of the free model-parameters
to the conditions in the relatively small lowland river catchment.
Using the standard data set, total nutrient emissions of 2320 tons year^-1 of nitrogen and 96 tons year^-1 phosphorus
were calculated. The detailed data set yielded slightly higher emissions for nitrogen (2420 tons year^-1) and for
phosphorus (102 tons year^-1). According to the spatial resolution, the proportion of the area of tile drainages and
sandy soils derived from the different data sets varies considerably. This causes great differences in the total nutrient
emissions estimated by the two approaches. Comparing the observed and the calculated nutrient loads, reliable results
for catchments larger than 50 km2, or third-order streams, could be shown.