Abstract
The Flemish region is densely populated, strongly industrialized and is characterized by intensive agricultural activities. These circumstances contribute to the long way to go to for a good water quality in Flanders. A huge step forward in environmental legislation was he implementation of the concept of integrated water management, in following of the European Water Framework Directive (2000/60/EG).
Integrated water management looks at water as a system and tries to synchronise the management of water quantity, water quality and life in the water environment.
Water quality models have proven to be a reliable tool for decision making and scenario analysis. It can be an aid for implementing specific measures towards the different target
groups in order to enhance the water quality in surface waters. In this study a SWAT (Soil and Water Assessment Tool) model is build for the Grote Nete, a subbasin of the Nete basin situated in the Scheldt catchment area. SWAT is a semi-distributed model which is capable of including both the diffuse and the point sources.
Starting from a well calibrated hydrologic model the calibration of the water quality model  in SWAT is carried out on a visual basis. Satisfactory results are obtained for O2, NO3 - and Nt. To become these fair results, two major adaptations were made in the SWAT source code. Phosphorus processes fail in the water quality model, so unsatisfactory results were obtained for phosphorus.
After set up and calibration, the model is used for scenario analysis in order to predict the effects of pollution load reductions for the different target groups. Point sources as well as diffuse sources are reduced by steps of 10%. Three scenarios are applied: reduction of the point sources, reduction of the diffuse sources and the reduction of both the point
sources and the diffuse sources. This scenario analysis is used to determine the reductions which are necessary to achieve standards, set by the Water Framework Directive.
Nitrate and BOD already comply with the norm, so no reductions are needed for these components. To achieve standards for DO and Nt, reductions of 20% are needed (point sources for DO and both the point and the diffuse sources for Nt). Since nitrate concentrations achieve the standard, the cause of Nt concentrations exceeding the norm
are probably the high concentrations for orgN and NH4 + discharged by unconnected households.
Model results from the Environment Costs Model (MKM) are used to carry out specific measures to achieve the desired reduction. For each measure the marginal cost is calculated. It is concluded that to achieve standard in the least expensive way, following measures must be implemented: establish highly productive dairy cattle, sowing of winter green cover, improve fodder efficiency pig's, implement basic measures and improve
WWTP efficiency. When applying these measures, 20% reduction is obtained. Also instream O2 concentrations will achieve standard when implementing these measures.

As households are responsible for not obtaining standards for total nitrogen it would not be fair to expect more efforts from agriculture, since nitrate standard is obtained. When the principle “the polluter pays” is applied, measures where households are
connected to the sewers and WWTP or are obligated to treat the wastewater their selves should be implemented. However, only 9.5% reduction is achieved this way and marginal costs increase extremely.

Appears in Collections:    IMM 2009