Storage water value as a signature of the climatological balance between resource and uses

Abstract. Water is stored in reservoirs to adapt in time the availability of the water resource to the various water demands like hydropower production, irrigation or ecological constraints. Deterministic dynamic programming retrospectively identifies optimal reservoir operations that could have been achieved to balance resource and demand during a given time period in the ideal configuration where future inflows and demand are perfectly known. A by-product of dynamic programming is the estimation of the storage water value (SWV) which is the marginal value of the future benefits potentially obtained from an additional unit of water volume stored in a reservoir and which determines the optimal storage strategy. The SWV depends on the reservoir level and shows seasonal as well as inter-annual variations. This paper uses the SWV as an index of the adequacy between water resources and water demands for a simplified water resource system in a mountainous region in France. It characterizes how and why the adequacy and optimal strategy could change for this system if the climate and/or demand change. Changes in mean regional temperature (increase) and/or precipitation (decrease) are analyzed. The influence of the nature of water demand on the SWV is also described (energy production or minimum lake level maintenance). In the studied case, the adequacy between water resources and demand either improves or degrades depending on the considered future scenario. In all scenarios, the seasonality of SWV changes with for example earlier water storage is to efficiently satisfy increasing summer water demand.