Simulation of the impacts of Shake River Plain aquifer water use on flow in the Snake River

Ground-water pumping on the eastern Snake River Plain and water use in the tributary valleys will ultimately affect flow in the Snake River. Spring flows to the Snake River have declined during the past two decades. The decline in spring and river discharge may be attributed to water-level declines in the Snake River Plain aquifer resulting from the combined effects of changes in aquifer recharge and increases in ground-water pumping. The effects of increased water use at selected locations in the Snake River Plain were estimated by simulating aquifer water use, spring flows, and river leakage using a ground-water flow model. The model was based on superposition concepts and designed to estimate only the changes in river flow resulting from ground-water use at specific locations in the aquifer. The model was constructed with an initially flat water table with four river reaches interconnected with the aquifer (e.g. fixed head nodes), but with no hydraulic gradient between the river and aquifer. Ground-water pumping was simulated at discrete locations and the response of river recharge and discharge was determined from computer modelling. Changes in flow are graphed in terms of a percentage of the rate of water use. Simulation results demonstrate how the river recharge and discharge in each of the four hydraulically connected reaches varies in response to ground-water use at different locations. Eighteen water use sites were simulated throughout the Snake River Plain. [...] The accuracy of the model predictions are affected by several factors. The degree of interconnection of the Snake River and Snake River Plain aquifer is not known with complete certainty in all areas. Local aquifer systems and additional surface water sources may be hydraulically connected with the Snake River Plain aquifer which are not included in the present simulation. Prediction accuracy is also compromised by errors in estimation of aquifer properties and numerical and discretization errors inherent with numerical simulation. The applied procedure and resulting graphs will further the general understanding of surface water and ground water interaction in the eastern Snake River Plain. The method allows evaluation of individual ground-water use impacts on several reaches of the Snake River