Three‐Dimensional, Transient, Saturated‐Unsaturated Flow in a Groundwater Basin

A three‐dimensional finite difference model has been developed for the treatment of saturated‐unsaturated transient flow in small nonhomogeneous, anisotropic geologic basins. The uniqueness of the model lies in its inclusion of the unsaturated zone in a basin wide model that can also handle both confined and unconfined saturated aquifers, under both natural and developed conditions. The integrated equation of flow is solved by the line successive overrelaxation technique. The model allows any generalized region shape and any configuration of time variant boundary conditions. When applied to natural flow systems, the model provides quantitative hydrographs of surface infiltration, groundwater recharge, water table depth, and stream base flow. Results of simulations for hypothetical basins provide insight into the mechanisms involved in the development of perched water tables. The unsaturated basin response is identified as the controlling factor in determining the nature of the base flow hydrograph. Application of the model to developed basins allows one to simulate not only the manner in which groundwater withdrawals are transmitted through the aquifers, but also the changes in the rates of groundwater recharge and discharge induced by the withdrawals. For any proposed pumping pattern, it is possible to predict the maximum basin yield that can be sustained by a flow system in equilibrium with the recharge‐discharge characteristics of the basin.