Elaboration et évaluation environnementale de scénarios prospectifs d’occupation des sols à l’échelle locale: Application au cas du ruissellement érosif dans le Pays de Caux, Haute-Normandie

Résumé Fr

For thirty years, the north‐western European loess belt regions (Pays de Caux, France) experience a resurgence of water erosion causing damages to agricultural land, but also in the valleys downstream where damages can be considerable,(muddy floods, turbidity of drinking water, roads deterioration). This increase is largely due to changes in land use and land cover, themselves being highly dependent on the changing political and economic context. Changes in land use and land cover contribute to a reorganization of cropping systems needing to be not only identified, but also assessed for their impacts on runoff. The main objective of this study is to develop a generic methodological approach to assess on one hand the influence of the changing context on land use and land cover and on the other hand the consequences of these changes in terms of runoff using biophysical models. This approach is based on the combination of 2015 local land use change scenarios built by the method SYSPHAMM with a model of spatio‐temporal allocation of crops to fields (LandSFACTS model) and two biophysical models for runoff assessment: one at the watershed level (STREAM) and the other at farm level (DIAR). Applied to the Pays de Caux (Seine‐Maritime), our approach is illustrated through the evaluation in terms of runoff of four scenarios derived from prospective, to assess the potential impact of the change in mediumterm context (2015) patterns of land use on runoff. The methodology presented suggests that assessing local land use scenarios in terms of runoff requires taking into account crop allocation diversity allowed by farmers’ decision rules. This requirement accounts for runoff variability at the watershed outlet since crop spatial distribution throughout the watershed, depending on farmers’ specific decision rules (i.e. cropping systems), strongly conditions runoff phenomenon. Besides, choices regarding scenario implementation (quantification and spatialization) should to be made according to those cropping systems. In terms of development of territories, the demise of dairy farming for instance would entails major consequences in terms of runoff volume produced at the watershed outlet, but potentially mitigated by the implementation of local actions in agricultural lands. The analysis of scenarios impacts on runoff showed the benefits of this type of approach to highlight the “scenarios at risk” on runoff and erosion and to identify levers for action for sustainable land management. In addition, the scenarios are a tool to facilitate consultation dialogue between local stakeholders, and encourage both local policy makers and local actors to actively discuss the future of land use in Upper Normandy.