Abstract
Human activities, such as stream management, drainage, urbanization and
agriculture, heavily influence the aquatic ecosystems in small lowland streams.
For the assessment of the impact of human activities of aquatic ecosystems, a
modeling tool is created. This modeling tool is part of the EU-life Dommel project.
The modeling tool consists of two parts: a
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model for the assessment of response
of aquatic ecosystems ‘ECOSTREAM’ and, described in this report, an aquatic
habitat condition model ‘STREAMFLOW’. The necessary aquatic habitat
conditions are flow velocity, saprobic state, stream dimensions and subdivision
between continuous and intermittent streams.
To comply with these habitat conditions, an environmental assessment tool was
needed. This tool had to be capable of performing hydrological operations,
calculate transport of water and solutes and perform compex GIS operations. The
dynamic GIS program PCRASTER, designed by the university Utrecht, complies
with this functionality. STREAMFLOW is build with cells of 500 by 500 meter
wide. Direction of transport is based upon height differences.
A waterbalance model is made, calculating stream discharge for every location in
the catchment (distributed), for time steps of 10 days over the period 1990 -
1996. The model is based upon the conceptual waterbalance model for the river
Rhine. Several discharge observations within the catchment indicate that
discharge simulations are reliable for most areas. In the direct neighborhood of
transfer of water between sub catchments or to canals, the hydrograph could not
be estimated appropriately. Results from the waterbalance are used for further
calculations.
Flow velocity calculations are based upon Manning's equation, needing stream
dimensions, the specific contributing discharge of every cell, the slope and a
roughness coefficient. The simulations yield for almost every cell into a flow
velocity above 0.10 m.sec-1; a distinct limit for aquatic ecosystems in flowing
water. The effect of weirs is implemented in the assessment model; a decrease
of the flow velocity just before the weir is simulated (often below 0.1 m.sec-1).
The calculated stream discharge and flow velocity are subsequently used for
assessment of the saprobic state. One way to define saprobic state is by means
of the concentration organic-N in streams. In STREAMFLOW, organic-N is
approximated by Kjeldahl-N. The major sources for Kjeldahl-N are households
and industry, disposal of effluent to wastewater plants, to untreated sewers and
direct to the stream. Degradation of Kjeldahl-N is accounted for as function of the
Kjeldahl-N load and residence time in a cell. After comparison with observations
at several locations in the Netherlands can be concluded that distributed pattern
of saprobic state is adequately simulated. The results of STREAMFLOW are
applied in the model ECOSTREAM (Olde Venterink et al., 1998).
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