An on-farm agronomic survey was carried out, at the field, farm and catchment levels, to understand the dynamics of the risk of soil erosion in a watershed displaying a rapid diversification of cropping systems and differentiation of family farms. The results were integrated into a researcher agent-based model linked to a geographic information system to represent dynamically the combined impacts of topography, rainfall and technical practices on erosion risk and its spatial distribution. A simpler version of this model was implemented as a role-playing game associated to a second agent-based model playing the game in silico, to share the researchers’ point of view on soil erosion dynamics to local farmers. They found these simulation tools relevant and Companion Modelling (ComMod) activities where launched with them to stimulate exchanges of experiences, build a shared diagnosis of the problem, and encourage collective action to limit soil degradation and the vulnerability of resource-poor holdings. The successive cycles of ComMod activities necessitated to complete the initial agronomic diagnosis by examining a series of farmer selected key questions. Gradually, local stakeholders moved towards directions not yet proposed by development agencies. The activities produced multiple effects related to awareness, self-confidence and communication, learning, change in practices and self-organization. The limited durations of successive training projects were not adapted to accompany local actors up to collective action at the catchment level, but new irrigation water and farm product processing infrastructures emerged a few years later in the village. The institutionalization of the approach would allow a more continuous companionship, facilitate the dissemination of results, and a move towards a more distributed governance of renewable resources.