Hydromorphic soils, hydrology and water quality : spatial distribution and functional modelling 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 control of nitrate fluxes by these areas, investigations were carried out on a detailed study site - the Coët Dan catchment (1200 ha) in Brittany, France - involving various disciplines: pedology, soil physics, hydrology, geochemistry and agronomy. An attempt of functional modelling at different hierarchical levels from the horizon level (i - 1) to the region 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 can 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 hydrodynamic properties, horizons of the soil system have been classified into “building blocks”, which allows to define physically based parameters for a two-dimensional multilayer water transfer model. A four compartment 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 several subcatchments were negatively correlated with the percentage of hydromorphic soils. These studies reveal that the hydromorphic zones had 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 designing new landscape management options.