Numerical evaluation of managed aquifer recharge as a conjunctive water resource management tool in the Walla Walla Basin
GeoSystems Analysis, Inc.
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The Walla Walla Basin, located in Eastern Oregon and Eastern Washington, USA, faces challenges in sustaining an agricultural water supply while maintaining sufficient flow in the Walla Walla River (WWR) to sustain endangered fisheries. Managed Aquifer Recharge (MAR) is currently used in the basin to supplement groundwater used for summer irrigation to allow greater instream flow during dry summer months. The numerical groundwater-surface water model, Integrated Water Flow Model (IWFM), was calibrated to hydrological conditions in the Walla Walla Basin and applied to predict future hydrological conditions under current management practices (baseline model) and for three alternative water management scenarios. Alternative management scenarios assumed unlined canals were converted into pipelines to improve the efficiency of irrigation water deliveries and a concurrent reduction of diversions from the WWR during summer months. MAR is incrementally increased among the three management scenarios, with “Current MAR” using the current annual MAR input of 11.1 million cubic meters per year (Mm3/Y) at the seven currently active MAR sites, “Increased MAR” using 17.7 Mm3/Y among 22 MAR sites, and “Maximum MAR” using 29.3 Mm3/Y among 60 MAR sites. Model results indicate that canal piping without increased MAR will increase instream flow in the lower portions of the WWR by up to 0.20 m3/s relative to baseline conditions. The predicted impact of MAR on instream flow is minimal in the upper portion of the WWR. Under the “Increased MAR” and “Maximum MAR” scenarios summer flows in the lower portions of the WWR are predicted to increase by up to 0.45 m3/s and 0.51 m3/s, respectively, relative to baseline conditions due to an increase in groundwater return flows in the WWR and tributaries. Conversion of canals into pipelines is predicted to decrease seepage from canals as a source of groundwater recharge, resulting in decreased groundwater storage in the “Current MAR” scenario relative to the baseline model. Under “Increased MAR” and “Maximum MAR scenarios groundwater storage was predicted to be greater than baseline conditions. Model results indicate that canal piping in combination with increased MAR provides benefits for riparian and instream habitat by allowing for significantly increased summer flows in the WWR and inflowing tributaries, while stabilizing groundwater storage levels. This supports that MAR is a tool that can be used to apply conjunctive water management effectively in the Walla Walla Basin.