Adjoint modeling to quantify stream flow changes due to aquifer pumping

November 2013.Includes bibliographical references.As populations grow and demand for water increases, new sources of water must be found. If groundwater resources are developed to meet these growing demands, the increased pumping of aquifers should not reduce flows in rivers to levels that would limit the availability of water for drinking water supply, irrigation, and riparian habitat. Stream depletion is the term for the change in the river flow rate due to pumping in an aquifer that is hydraulically connected to the river. In many regions of the U.S., a new well cannot be sited until it is shown that pumping the new well will not cause substantial stream depletion. Numerical simulations are typically used to quantify stream depletion. In the standard approach, two numerical simulations are run—one without pumping and one with pumping in a well at the proposed location. In both simulations, the water flux between the river and aquifer is calculated, and the difference between these fluxes is the stream depletion due to pumping at the proposed well location. If multiple well locations are considered, one addition simulation must be run for each additional potential well location; thus, this approach can be inefficient for siting new wells. The goal this research was to develop an adjoint-based modeling approach to efficiently quantify stream depletion due to aquifer pumping. In a single simulation of an adjoint model, stream depletion is calculated for a well at any location in the aquifer; thus, it is computationally efficient when the number of well locations or possible well locations is large. The adjoint approach was developed to be used with standard groundwater flow simulators, and therefore can be applied in practice. The research included rigorous development of the adjoint equation for calculating stream depletion in confined and unconfined aquifers with various models of groundwater/surface water interaction, along with numerical simulations to verify the adjoint equation. In addition, we used the adjoint method to investigate the sensitivity of stream depletion to the hydraulic conductivity of the stream channel, a parameter which is known to be uncertain