SCALES : a large-scale assessment model of soil erosion hazard in Basse-Normandie (France)
Résumé
The Basse-Normandie is regularly affected by Rill-Interrill erosion system whose origin is mainly related to the agricultural practices. The fight against erosion is registered like a priority in the policies of regional management which seek tools for diagnosis of fine scale of soil erosion hazard. For this reason, the Department of Calvados (Basse-Normandie) wished to obtain a tool to identify the areas of production of erosion on the whole of its territory (5500 km2). The SCALES model (French acronym which means Spatialisation d'eChelle fine de l'ALéas Erosion des Sols) which we propose registers from this point of view. It concerns the research tasks relating to the mapping of the potential susceptible areas and the soil erosion hazard. To define the potential susceptible areas, we take into account soil erodibility, agricultural land cover and topography. Soil erodibility is defined by structural instability, agricultural land cover by grasslands and croplands, topography by slopes. Croplands are defined by crop rotations (4 years period) and intercrops management. Soil erosion hazard results from combination of potential susceptible areas and rainfall erosivity. This one is defined by rainfall daily intensity and yearly positive hydrological balance. Which are the databases used? For rainfall erosivity, we use a departmental meteorological pattern which represents one station for 200 km². Database concerns monthly rainfall, monthly potential evapotranspiration, yearly and monthly number of days with a daily intensity is above 10 mm and yearly and monthly positive hydrological balance. For soil erodibilty, the existing informations are very poor. So, we had to create our own database. We had to realize 6000 soil bareholes in order to know the soils. Pedogenesis, soil thickness, constituents and organisation have been characterized. Analytic assessments of representative soil boreholes have been realized for representative soil boreholes. Physical, structural, hydrologic and chemical properties have been specified. For agricultural practices, firstly, we had to make a departmental typology of agricultural specialities of farmer exploitations. From investigation in farmer exploitation types, we constituted, on departmental scale, a spatial and temporal database concerning crop rotations and intercrops management . About topography, slope properties are derived from a DEM with a grid resolution of 20 m. SCALES model is a large-scale assessment model. That supposes an integration of soil erosion hazard components in Elementary Integrated Spatial Units corresponding to the elementary «Graphic parcels». Graphic parcels exist in digital format and have been derived from the Inventory of Common Agricultural Politics. For Department of Calvados, that represents 28 000 grassland graphic parcels and 60 000 crop graphic parcels. Assessment of potential susceptible areas and of soil erosion hazard results from the following step. Input data of the components which define the potential susceptible areas and the soil erosion hazard are converted into levels of pressure (level 0 : no pressure, level 5 very strong pressure). According to additional reasoning, levels of pressure from input data are combined. A simplified classification of added levels is proposed in terms of combined levels of pressure. The additional reasoning is generalized for all combined levels of pressure and allows to assessment rainfall erosivity and potential susceptible areas. Combination of rainfall erosivity and potential susceptible areas allow to define different levels of soil erosion hazard. Whatever the levels of others input data, grassland parcels and parcels for which slope is lower than 1% are characterized by level 0 concerning soil erosion hazard. SCALES is a hierarchic model with organization of input data and combined data. It is used for yearly assessment of soil erosion hazard but it can be also used for seasonal assessment. Some input data present a seasonal evolution, that influence combined levels. The integration of these evolutions allows to carry out seasonal maps. This step can make it possible to work out simulations in the most significant sectors in order to consider a follow-up on a temporal large scale. This tool will allow to develop a monitoring concerning the potential surfaces of soil erosion production and to start procedures of actions in order to limit the consequences beyond transmitting surfaces. Lastly, SCALES must allow to set up local action plans concerning application of the European Water Framework Directive.