A coupled surface water and groundwater flow model

In hydraulically well-connected groundwater and surface water systems, stream-aquifer interaction has been simulated using a MODFLOW model developed by USGS, which couples hydraulic behavior of groundwater and surface water subsystems. It assumes a constant stream-stage during each stress period, employing a stream flow routing module which is limited to steady flow in rectangular, prismatic channels. One of the challenges in understanding the interaction of surface water and groundwater systems lies in their different time scales. In this paper, the INTRACT model is developed and incorporated into MODFLOW.  INTRACT simulates unsteady, nonuniform flow by solving St. Venant equations. Terms that describe leakage between stream and aquifer as a function of streambed conductance and differences in water table and stream stage were incorporated into the continuity equation. INTRACT calculates new stream stages for each time step in a transient simulation based on upstream boundary conditions, stream properties, and estimated head distribution. Next, MODFLOW calculates head distribution using aquifer properties, stresses, and stream stages calculated by INTRACT. This process is repeated until convergence criteria are met for aquifer head distribution and stream stages. Because the time steps used in groundwater modeling can be much longer than time intervals used in surface water simulation, a provision has been made for handling multiple INTRACT time intervals within one MODFLOW time step. Performance of the coupled model was validated using an analytical solution from the previous studies