The International Conference on the Protection of the North Sea (ICN) and the Helsinki
Commission (HELCOM) agreed on a 50 % reduction in nitrogen and phosphorus inputs to the
North Sea and Baltic Sea respectively for the periods 1985/1987 to 1995. The fulfilment of
this agreement is a political necessity in relation to sea conservation. In the next two decades,
water quality guidelines will be put in place for the reduction of loadings to inland and marine
waters to a satisfactory ecological state.
With this in mind, it needs to be tested as to what extent the nutrient loadings of the North and
Baltic Seas from German freshwaters have been further reduced since 1995. The basis of this
work was the model MONERIS developed by Behrendt et al. (1999). This model provides an
overall as well as catchment-differentiated quantification of the input pathways. The
catchment-differentiated analysis allows the identification of regional lading hot spots which
is important for the implementation of measures for the reduction of nutrient inputs.
The nutrient loading of freshwaters is from both point source and diffuse source input. Since
knowledge on the contributions of the individual input pathways in particular catchments is
necessary for the determination of further measures for the reduction of nutrient loadings, the
emission calculation model was employed based on the studies of HAMM (1991) and WERNER
& WODSAK (1994). The comparability of the results of HAMM (1991) for the old German
states (former FRG) and of WERNER & WODSAK (1994) for the new German states (former
GDR) was, however, hindered by the use of different methods and study periods. The analysis
of BEHRENDT ET AL. (1999) unified the various methods, and the results were catchment
oriented as the input pathways are related to discharge components.
In the context of previous studies, not all existing problems can be solved relating to estimation
of nutrient inputs and loadings to catchments and their changes over time. The aim is to calculate
nutrient inputs to German catchments for the time around 2000, to test whether estimates of
inputs by various pathways to catchments in neighbouring countries can be applied to Germany
and to compare methods. With regard to this, harmonised model parameters should be employed
for determination of nutrient inputs within the existing model and through the enlarged database,
the model should be improved.
In hindsight, a comparison of model results makes it possible to harmonise inputs as well as to
use similar or identical calculation methods, unified databases, definitions and linkages.
The catchment based analyses of nutrient loadings should provide a final picture for all areas,
from the causes of the loadings to the realised carriage. In this regard, a catchment based
nutrient balance for agricultural land for the period 1985 to 1999 is necessary for it is the
relationship between this balance and the various above and below diffuse input pathways in
dependence on the geomorphological, soil and hydrological conditions in particular catchments. The methodological work in the present analysis concentrates predominantly on
the relationship of the most recent knowledge on regional differences in the underground
retention times in the model calculations. However, some important improvements in details
of the model were also made to reduce existing discrepancies. As before then, the goal of the
study is not only to estimate catchment differentiated nutrient input but also to calculate
nutrient loadings so that model results can be directly compared with observations.
The transfer of the pathway-based and catchment-differentiated model results to catchments
outside Germany was possible with use of information on the Odra and its sub-catchments
(Behrendt et al., 2002). From this, the usability of the model in other catchments (Danube,
Vistula, Po, Axios etc) is currently being tested in various national and EU projects.
Since the model inputs of MONERIS were originally designed to be used for catchments of
500 km 2 and larger, the need of an analysis of the loading situation in conjunction with water
framework directive (WFD) for catchments of 100 km 2 or less is necessary. It is a goal of this
and other studies to test whether the model can be applied for such small catchments. This is
being carried out in contracts for the states of Baden-Württemberg and Brandenburg
(Behrendt et al., 2000, 2001) as well as four graduate studies (Venohr, 1999; Geisler, 2001;
Thomas, 2001; Schmidt 2002).