Soil erosion hazard map for river basin managers: An example for the water bodies of the Loire river basin (France)
Résumé
Soil erosion on agricultural land is associated with deleterious off-site impacts including the siltation and the pollution of the receiving water bodies. To better manage this situation, local/regional water agencies need spatially-distributed information to compare the sensitivity to erosion of the areas draining into these water bodies and supplying the vast majority of sediment and associated pollutants leading to this water quality impairment. These soil erosion hazard maps are now often included in the latest versions of the basin management plans that must be designed to meet the water quality objectives of the EU Framework Directive. However, the resolution of these maps is often too coarse to meet the practical needs of these agencies. Accordingly, the current research used the latest input databases to improve the MESALES model outputs in one of the largest French River basins (Loire, 117,000 km2), with the implementation of three main modifications. First, the seasonal variations of land cover were incorporated into the model through a revised set of expert rules based on the agricultural census data. Second, the discrimination of the soil textures was improved within the infiltration and erodibility module of the model. Third, variations in rainfall erosivity across the study area were described taking into account the latest erosivity map available at the European scale. Then, the model results obtained with the updated model version were compared with those generated by the previous version. Overall, the simulated soil erosion hazard changed for 35% of the pixels of the Loire River basin, with a significant increase of the lowest hazard classes during all seasons except summer. When aggregating the results at the scale of water bodies, the simulated erosion hazard changed for 49% of these management units. Although 28% of these water bodies were associated with a lower hazard, 23% of the river systems were attributed a higher erosion risk. The implications of these model/map revisions for the local decision makers were discussed, taking the strategy of concentrating the management budget on those water bodies associated with the highest erosion risk as an example. In the future, this model could be used to compare the soil erosion hazard in contrasted regions of Europe and to simulate the impact of management plans designed to decrease this risk to support the decisions of water agencies.