HYDROMORPHIC SOILS, HYDROLOGY AND WATER-QUALITY - SPATIAL-DISTRIBUTION AND FUNCTIONAL-MODELING AT DIFFERENT SCALES

The hydrology and water quality in landscapes with hydromorphic soils depends on the space and time extension of wetland areas and on water pathways within the landscape at different scales. To study the contro l of nitrate fluxes by these areas, investigations were carried out on a detailed study site - the Coet Dan catchment (1200 ha) in Brittany, France - involving various disciplines: pedology, soil physics, hydro logy, geochemistry and agronomy. An attempt of functional modelling at different hierarchical levels from the horizon level (i-1) to the reg ion level (i + 3) of soil distribution, extension of saturated areas, horizons physical characteristics, water transfer in a multilayer soil profile and nitrate fluxes was carried out. The soil system, which ca n be described as a spatial arrangement of a limited number of horizon types with genetic relationships, is tightly controlled by topography . Predictive models of hydromorphic soil distribution using different topographic indexes and DEM were established. Regarding to their hydro dynamic properties, horizons of the soil system have been classified i nto ''building blocks'', which allows to define physically based param eters for a two-dimensional multilayer water transfer model. A four co mpartment model of flood genesis based on chemical data obtained from different parts of the catena and from the river was coherent with the multilayer hydrodynamic model. The mean nitrate concentrations in sev eral subcatchments were negatively correlated with the percentage of h ydromorphic soils. These studies reveal that the hydromorphic zones ha d an effect on the nitrogen transfer in the catchment, but this effect is limited by the importance of water pathways by-passing the buffer zones. The conclusions of this programme have direct outcomes for desi gning new landscape management options.