Numerical ground-water flow modeling of the Snake River Plain aquifer using the superposition technique

Predictive ground-water flow modeling may be simplified by application of superposition when the governing equations are linear. The simplification allows evaluation of impacts of individual aquifer stresses and minimized model input, output, and interpretation, Modeling is performed by using (1) boundary conditions and aquifer properties provided by previous calibrations or analytical techniques, (2) setting the initial potentiometric surface and prescribed-head boundaries to an arbitrary horizontal datum, and (3) simulating a specific recharge or discharge stress. Superposition was applied to an existing, calibrated model of the Snake River Plain aquifer to simplify prediction of changes in interaction with the Snake River, Simulations predict the temporal relationships between ground-water use at multiple locations within the Snake River Plain and surface-water depletion in four hydraulically connected reaches of the Snake River. Simulated aquifer water use at a location approximately five miles from a hydraulically connected river reach results in river depletions greater than 80% of the pumping rate after 10 years. Water use further than 50 miles from hydraulically connected river reaches results in depletions from 10 to 30% of the annual average pumping rate after 100 years, Results present spatial and temporal impacts of water uses on the Plain that are conceptually and quantitatively beneficial to water resources planners and water users.