Nitrate transfer from agricultural sources via river networks remains a serious unresolved and complex issue. This article proposes an economic analysis of the optimal reduction of this nitrate. A linear transformation and transport model of nitrogen inputs from agricultural sources in the form of nitrate from five agricultural areas towards a hydrographic network in France is used to calculate the optimal effort to reduce nitrogen inputs on the basis of a cost-benefit analysis (CBA). A sensitivity study is implemented with different damage scenarios. In addition, efforts to reduce uniform and spatialized inputs are compared. In particular, our results show the determining role of the magnitude of the damage. The ratio of 1 to 3 between the low and high range of its estimation would make it possible to attain good status, as specified by the Water Framework Directive (WFD), without having to resort to the exemption procedure, decreasing the average optimal nitrate concentration from 47 mg/l to 42 mg/l. Moreover, this would increase the absolute and relative benefits of spatialization by a factor of 9 and 2, respectively. •Optimal nitrate reduction for a catchment is calculated using a cost-benefit-analysis.•Uniform and discriminating reductions of agricultural nitrogen are compared.•The impact on the results of the different damage-related hypotheses is discussed.•The hypothesis concerning the global damage level plays a crucial role in the results.•Greater damage magnitude tends to increase the advantage of spatialization.