S. Even

River ecosystem modelling: application of the PROSE model to the Seine river (France)

The Seine river crosses the most densely populated and industrialized area of France, Paris (16 million inhabitants), surrounded by fertile land with intensive agriculture. In the framework of a CNRS (Scientific Research National Center) research project, computer programs have been designed to tackle problems related to eutrophication, non-point pollution and the impact of sewage during dry or wet periods (urban runof and sewage network overflow). The PROSE software has been specially designed to simulate the behaviour of the most disturbed stretches of the Seine ecosystem on the last 300 kilometers of the river, upstream of the estuarine area. The 1-D hydraulic sub-model of PROSE is based on a finite difference solution of Saint-Venant equations solved with the Preissman scheme. It simulates steady state situations as well as highly transient situations such as fast changes in river discharge during rainy periods or dam motions. The biological sub-model is based on the RIVE model, describing the major processes in a river ecosystem: primary production, heterotrophic bacterial activity and organic matter decomposition, major nutrients species (nitrogen, phosphorus), nitrifying activity and oxygen balance. Water column and sediment variables are simulated. Most of the parameters have been estimated during laboratory experiments or field studies. Different situations observed between 1989 and 1991 allowed a detailed validation of the model. The model was then used to explore the reaction of the ecosystem (particularly its oxygen status) to changes in physical constrains (discharge, reoxygenation at dams) or in biological processes (release of microorganisms accompanying waste water discharge).

An integrated modelling approach to forecast the impact of human pressure in the Seine estuary

Within the framework of the European Water Framework Directive, the Seine-Normandie Water Agency has defined prospective scenarios describing possible trends of evolution of the pressures on water resources. In order to evaluate the resulting water quality improvement or degradation of water bodies in the Seine river basin, an integrated modelling was proposed. The approach consisted in coupling three models, the seneque model for upstream sub-basins, the ProSe model for the Seine river and main tributaries and finally the siam1d model for the downstream estuary. After consistency verification, the integrated model was applied to scenarios proposed by the Seine-Normandie Water Agency. As a result of improvement in the nitrogen treatment by waste water treatment plants, the annual load of ammonia at the basin scale will be reduced by 65%. The oxygen and ammonia criteria in the estuary will improve from “bad” to “good”. However the nitrate criteria will remain “poor”, given the strong influence of non-point sources. Despite a 70–75% drop of the point orthophosphate loads, the criteria for this variable will also remain “poor”. The nutrient levels will be high enough to maintain eutrophication in the system; a general trend to a shift from N-limitation to P-limitation will be accentuated.

Hydrological part of CAWAQS (Catchment Water Quality Simulator): fitting on a small sedimentary basin

Integrated water resources management tools require considerab1e expertise and are data intensive. Different processes have to be taken into account: hydro1ogy, transport and biogeochemistry. Estimated inputs depend on avai1ab1e information and data. The catchment is typically divided into different compartments, and processes are simulated with a conceptua1 or a physically based mode1 (ARHEIMER & ÜLSSON 2003). Extensive work has been carried out to bui1d catchment based mode1s for 30 years, such as for SHE (ABBOTT et al. 1986) and SWAT (NEITSCH et al. 2002).

Modelling the carbon cycle in the turbidity maximum of the Seine estuary

Estuaries are characterised by long residence time (CIFFROY et al. 2003, ), accumulation of high amounts of suspended matter (SM) in the turbidity maximum zone. They receives the waters of the whole drainage basin before transfer to the coastal and oceanic domains. Thus the question of the role of the estuaries for transformation, production and exportation of nutrients and organic matter was regularly renewed (DAY et al. 1989, ) and, given the complexity of estuaries and contexts, remains largely discussed (ABRIL et al. 2002, ). Given the difference between dissolved and particulate species transport, particulate organic matter fate is essentially under concern (KNOX 1986, ). This question was studied in the framework of SEINE AVAL, research program dedicated to the Seine estuary. In order to determine the relative importance of the sources of POC in the estuary and to interpret the high organic carbon (POC) content of the suspended matter (SM) in the turbidity maximum (up to 5 %) (ABRIL et al. 2002, ), a modelling of the carbon cycle was undertaken, based on in situ and laboratory measurements.