This article establishes the conditions for the distribution of a water flow among different users using the concept of aggregate economic efficiency. The complexity involved in the assessment of non-consumptive water uses was found to limit the problem of allocation to the implementation of a second-best solution. We conclude that the charging a rate based on an annual water scarcity index that results from a monthly average is an implementation error of environmental authorities which is detrimental to the preservation of the ecological water flow, and could also affect users downstream. Finally, it proposes a method for estimating the fee rate.