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International audience; Unlike many structuring projects, treated wastewater reuse projects are rarely subjected to economic analysis. And when they are, the social and environmental benefits and costs are often not accounted for or are not properly quantified. Here we show that the widely used cost-benefit analysis method showing the profitability of a project from the whole of the community's point of view can be adapted to treated wastewater reuse projects. The remaining evaluation difficulties are more due to the system's complexity rather than its methodological limits. Indeed, the operator must be able to understand, formalize and imagine constraints and risks associated with different domains such as urbanism, agriculture, climate, hydrology and trade. Remaining uncertainties over key parameter values can, however, be properly considered through scenarios and/or stochastic approaches. We illustrate the implementation of this methodological approach with two case studies: Clermont-Ferrand where treated wastewater is reused by a collective irrigation network; and Cannes where surface and groundwater recharge can enable indirect reuse for multiple purposes including indirect potable water production. These two French case studies highlight how economic analysis, dealing with uncertainties, can support decision making.; Les projets de réutilisation des eaux usées traitées font rarement l’objet d’analyses économiques. Lorsque c’est le cas, les coûts et bénéfices sociaux et environnementaux ne sont pas correctement pris en compte ou quantifiés. Nous montrons ici que la méthodologie d’analyse coût–bénéfices (ACB) permettant d’évaluer la rentabilité d’un projet à l’échelle d’un territoire peut être adaptée à l’évaluation de projets de réutilisation des eaux usées traitées. La méthodologie employée est bien maitrisée et les difficultés de mise en oeuvre sont davantage liées à la complexité du système. L’opérateur doit en effet être en mesure de comprendre, de formaliser et d’imaginer les contraintes et les risques associés à différents domaines tels que l’urbanisme, l’agriculture et l’hydrologie. Les incertitudes résiduelles sur les valeurs de paramètres clés peuvent être prises en compte par l’étude de différents scénarios et/ou par des approches stochastiques. Nous illustrons cette approche à travers les études de cas français de Clermont-Ferrand où des eaux usées traitées sont réutilisées par un réseau collectif d’irrigation, et de Cannes où la recharge de milieux permet la réutilisation indirecte pour différents usages, dont la production indirecte d’eau potable. Ces études de cas illustrent comment l’analyse économique peut aider à la prise de décision.

Irrigated agriculture is often criticized for its poor resource-use efficiency in the case of sprinkler systems in comparison with localized irrigation. Water losses due to evaporation of droplets during flight and wind drift of droplets out of the target area are often generalized regardless of the context in which they were measured, and may reveal the evaporation capacity of the measurement facility rather than that of the irrigation system itself. Using an approach combining in situ measurement (rainfall, electric conductivity) and simulations, losses were estimated according to droplet size distribution. Droplets less than 1 mm in diameter displayed the highest losses. Losses due to evaporation represented 30-50% of total loss; the remaining 50–70% were due to wind causing small droplets to drift outside the target zone. With a Rain Bird 46 sprinkler, total losses for the different periods of the day should not represent more than 20% at any time, or less than 4% for a 24-h irrigation period in the climate conditions that prevail in summer in the south of France. Copyright © 2011 John Wiley & Sons, Ltd.