Reducing the computational cost of unconfined groundwater flow in conjunctive-use models at basin scale assuming linear behaviour: The case of Adra-Campo de Dalias

The optimization of conjunctive-use management models (with different competing demands, several reservoirs and aquifers, and a high interconnection among elements) over long time horizons requires the use of computationally-efficient groundwater flow models. A review of conjunctive use models and some methods applied to reduce the computational cost of the groundwater flow model has been performed, with special attention to the assumption of linear behaviour. The effect of considering invariant transmissivity in a conjunctive-use management model at the basin scale has been analyzed using as case study the Adra-Campo de Dalías system (Southeast of Spain). The system includes an unconfined aquifer not hydraulically connected to the river in which significant drawdown, relative to the saturated thickness, can appear. The basin scale system also includes storage in a reservoir, flows in several river segments and canals, and a second simpler aquifer. The management formulation seeks the best conjunctive operation of reservoir and aquifer to meet urban and irrigation demands while satisfying minimum flow and aquifer head constraints over a time horizon of several years. The effect of the linear assumption has been studied by comparing and analyzing management model results obtained modeling the aquifer as confined and as unconfined. Groundwater flow is simulated in the optimization model using the response matrix method. The optimization problem is solved using sequential linear programming (SLP) in the unconfined case and using linear programming (LP) when confined behaviour is assumed. Although the drawdown are significant, the difference in results is minor, suggesting that the use of a linear assumption may be valid in many basin scale conjunctive management problems in which the aquifer is not hydraulically connected to the river system. Nevertheless, the assumption should be tested, if possible, in each management problem with unconfined aquifers in which significant drawdown appears