Evaluating Suitability for Surface Managed Aquifer Recharge to Enhance Dry-Season Stream Flows

Surface managed aquifer recharge (MAR) can enhance dry-season streamflow, but it requires sufficient infiltration through the unsaturated zone to generate a groundwater mound, and the mound must dissipate and increase groundwater discharge to surface water during the targeted time. We use multi-criteria decision analysis (MCDA) at state-wide and local scales to evaluate the potential for surface MAR in the upper Big Hole valley in southwest Montana, where the objective is to infiltrate water during snowmelt to increase streamflow several months later. Using criteria constraints, which remove areas that receive a low rating for any criterion, eliminated 91% of the area since most of the "medium" suitability areas resulted from averaging high and low-rated criteria. We added a distance-to-surface-waters criterion based on modeling the time required for flow through the unsaturated zone (HYDRUS-1D) and the timing of increased groundwater discharge to surface water (MODFLOW 2000). To achieve a 2-to-4-month lag time, reasonable distances for infiltration from surface water were about 600 to 1800 m. Groundwater discharge to surface water was substantially buffered relative to infiltration; even at optimal distances the rate of increased groundwater discharge to surface water during the target period was only about 15% of the infiltration rate. Adding the distance criterion further reduced the area suitable for surface MAR to about 4% of the total area. A phased approach during MAR-based suitability analyses is important for improving site feasibility interpretations, applying project-specific objectives, focusing resources, and retaining clarity and flexibility for collaborators and decision-makers.