Managing correlated stock externalities: water taxes with a pinch of salt

This paper addresses the design of corrective taxation schemes to manage multiple, interacting stocks. In the setting of irrigation-induced salinity, a collective groundwater stock aliments a set of individual rootzone salt stocks. It is shown that the policy maker does not need to base the tax rule on the individual salt stocks. Indeed, taxes based on the level of input and on the collective stock induce the agents to achieve the socially optimal irrigation path. An even simpler instrument is sufficient if the policy-maker's goal is to reach optimality at the steady state only.