Streamflow, a GIS-based Environmental Assessment Tool for Lowland Streams

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 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).