Implementing the Water framework directive requests that regulator at local scale can choose among all potential measures those that are the most effective for water pollution mitigation. On the watersheds, the pollution in water is the result of point and non-point sources. For the latter, the difficulty in measuring the individual emissions renders the design of mitigating policies particularly uneasy. When facing heterogeneous agents, the regulator can choose to apply uniform instruments (these instruments are non-contingent to the heterogeneity of the farms). To mitigate NPS nitrate pollution, these instruments can be taxes on N fertilizers, quotas on N fertilizers, or uniform reductions of the production level. On the other hand, the regulator can build differentiated regulations to take into account the agents' heterogeneity. These regulations will induce self selecting constraints, for two reasons. First, the agent's type can be unobservable. Second, even with observable types, institutional constraints prevent from first degree discrimination. This chapter contains guidelines to compare good farming practices at the watershed scale. It focuses on main key elements useful for decision making (BMPs and critical areas definitions, cost/effectiveness approach, acceptability and integrative analysis grid).