Economic optimization of surface and groundwater conjunctive use in a water resources system: Contribution to the economic analysis proposed in the European Water Framework Directive (Spanish text)

A methodology for optimizing conjunctive use of surface and groundwater at the river basin scale is presented. Integrated river basin modeling, including distributed simulation of groundwater flow and dynamic stream-aquifer interaction, allows a more realistic representation of conjunctive use and associated economic results. The use of an economic objective function consisting in maximizing the net economic value of water use in the system allows generation of solutions that optimize the economic efficiency in water management. The inclusion of the appropriate constraints guarantees the feasibility and sustainability of the policies generated.; The economic characterization of water uses is based on convex water scarcity cost penalty-functions, obtained by integration of economic demand curves. Groundwater hydrology is embedded using the Eigenvalue Method. Parameter-distributed groundwater flow simulation allows dynamic estimation of pumping costs, stream-aquifer and sea-aquifer interaction, and screening of the aquifer's evolution. Constraints to aquifers' exploitation can be included. The analytical formulation of the Embedded Multicellular Models offers a simple and parsimonious approach to model stream-aquifer interactions in those cases in which no calibrated distributed groundwater model is available.; The optimization of the hydrologic-economic model offers a great variety of results, useful for a preliminary search of possible strategies to improve water management and system operation, and to plan system's capacity expansion. Net benefits of water allocation flexibility and optimal conjunctive use can be assessed by comparing the results of the optimization models with the results of a simulation model that reproduces the actual operating rules of the system. The European Water Framework requires estimation of resource and environmental costs. Shadow values associated with constraints for environmental purposes (like minimum streamflows) provide the marginal opportunity costs of those constraints. Shadow values on balance constraints provide the marginal value of water at that location, which can be used as an estimator of the resource cost. Shadow values on storage and conveyance capacity constraints reveal which elements constraint a more efficient water allocation and the additional benefit from increasing those capacities. A nonlinear optimization model has been programmed in GAMS to demonstrate the applicability and potential of the approach in a study case.