Unsteady groundwater flow beneath strip oceanic islands

Two one‐dimensional models based on the Dupuit‐Forchheimer assumptions to simulate flow beneath strip oceanic islands are presented. A model which is here designated the continuously moving interface (CMI) model is solved by finite differences. The solution agrees well with an analytical solution for a particular case given by Collins (1976). Because of its long response time the CMI model is not able to simulate short‐term fluctuations in water level in a well located on the South Fork of Long Island, New York. A second model, designated the delayed interface response (DIR) model, can reproduce the average hydrograph of the well and the decline in water level during most of an extended drought. However, because the model requires discontinuous movement of the interface between freshwater and salt water, some error is introduced into the solutions. The DIR model is used to predict the response of water levels to modifications in recharge patterns on the South Fork of Long Island.