This paper deals with policy mechanism designs for agri-environnemental schemes when the bio-physical processes are characterized by threshold effects. There is a threshold effect when specified farming practices must be applied on a minimal share of an area of interest to trigger perceptible changes of the state of the natural environment. Schemes result in a pure economic loss if the induced agro-environmental efforts are not sufficient. Different situations are considered including the lack

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... f information on farmers' characteristics or actions, uncertainty on the relationship between farming practices and environmental quality, and combined difficulties of scheme design.