A general methodology to simulate groundwater flow of unconfined aquifers with a reduced computational cost

The computational cost of groundwater flow simulation can be crucial when analyzing complex conjunctive use water resources systems that need to simulate simultaneously surface and groundwater components. A general methodology for accurate simulation of unconfined groundwater flow with low computational cost is presented. It requires linearizing the unconfined groundwater flow problem governed by the Boussinesq equation. The technique is based on a change of variable and depends on the reference level adopted. Some recommendations have been provided to set the reference level to estimate the spatially variant parameters required to define the linearized problem. Using this linearization, more accurate results can be obtained than those derived with the classical assumption of invariant transmissivity. Solving the problem with eigenvalue techniques, the solution can be defined with a semi-explicit state equation with low computational cost. Some case studies have been analyzed in order to demonstrate that the methodology can be applied to any aquifer geometry (including non-horizontal bottoms), hydrodynamic properties and boundary conditions (even different prescribed head values). The results have been compared with those obtained with other linearization methods and MODFLOW [McDonald, M.G., Harbaugh, A.W., 1988. A modular three dimensional finite difference ground water flow model. Open – File Report 83-875, US Geological Survey, Washington DC] for unconfined aquifers. A case study defined from a previously calibrated finitedifference model of the “Delta Adra” aquifer, located in southern Spain, has been also analyzed.