Coping with Urban & Agriculture Water Demand Uncertainty in Water Management Plan Design: the Interest of Participatory Scenario Analysis

Managing water scarcity is a major challenge for regions all over the world. In the European Union, robust methodologies are needed to establish effective programmes of measures aimed at achieving the "good status" of water bodies according to the Water Framework Directive (WFD). These programmes often target the current gap between the actual status of water bodies and the "good" status without accounting for uncertainty in water demand. We develop a new methodological framework that enable to account for uncertainty in future water demand and design programmes in order to increase their likelihood of attaining the good quantitative status. The foresight approach enables to construct and quantify future water demand scenarios hand-in-hand with stakeholders during workshops. They consist in identifying drivers, debating pre-constructed scenarios, reconstructing scenarios and estimating water demand. The impact of the co-constructed scenarios is simulated with a resource-demand balance model for all water resources and a cost-effectiveness analysis makes it possible to construct programmes that target the estimated future water deficits at least cost. The methodology is illustrated with an application to Reunion Island (Indian Ocean, France) considering agriculture (Ag) and urban water (Uw) demand. Three combinations of sector scenarios (Uw, Ag) were produced and coherence was eventually ensured by fitting the land use parameter. This solution can accommodate case studies faced with a binding land constraint for housing and agriculture. As each scenario implies significantly different programmes of measure in terms of intensity and spatial distribution, results demonstrate the importance of taking uncertainty on water demand into account.