Representing Pump-Capacity Relations in Groundwater Simulation Models

            The yield (or discharge) of constant-speed pumps varies with the total dynamic head (or lift) against which the pump is discharging.  The variation in yield over the operating range of the pump may be substantial.  In groundwater simulations that are used for management evaluations or other purposes where predictive accuracy depends on the reliability of future discharge estimates, model reliability may be enhanced by including the effects of head-capacity (or pump-capacity) relations for pumps on the simulated discharge from the well.  An algorithm has been incorporated into the widely used MODFLOW groundwater flow model that allows a model user to specify head-capacity curves.  The algorithm enables the model to automatically adjust the pumping rate each time step to account for the effect of drawdown in the cell and changing lift in the well, and will shut the pump off if lift exceeds a critical value.  The algorithm is available as part of a new multinode well package (MNW2) for MODFLOW.