The groundwater flow equation is written in curvilinear coordinates whose coordinate surfaces coincide with the top and bottom surfaces of folded layers of aquifers. The coordinates are general enough for these surfaces to coincide for almost all groundwater systems. The terms of the finite-difference approximation for the flow equation can be separated into two groups. One group corresponds to a similar system of horizontal aquifers and the other group corresponds to the folding. The latter group is zero if the vertical gradients of hydraulic head and hydraulic conductivity are zero. Furthermore, it is noted that vertical gradients in head must be modelled for the effects of folding to result. When these vertical gradients and dips are not too large, the effects of folding can be calculated using a perturbation method wherein the flow in folded aquifers is considered to be a perturbation of the flow in similar horizontal aquifers. With the method presented, three-dimensional finite-difference models can be modified to simulate folded aquifer systems. Perturbation solutions are obtained for a class of folded three-aquifer systems. For these systems, with vertical head gradients as great as 0.23, with aquifer hydraulic conductivities differing by two orders of magnitude, and with dips as great as 30??, only small hydraulic changes due to folding were simulated. ?? 1985.
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Evaluating the hydraulic effects of changes in aquifer elevation using curvilinear coordinates