Modeling the impact of aquifer recharge, in-stream water savings, and canal lining on water resources in the Walla Walla Basin

The Walla Walla Basin in Eastern Oregon and Washington, USA, faces challenges in sustaining agricultural water supplies and endangered fisheries in the Walla Walla River (WWR). 11.1 Mm(3)/year of managed aquifer recharge (MAR) is currently used in the basin to supplement groundwater with the goal of maximizing instream flow during dry summer months. A numerical groundwater-surface water model was calibrated to observed hydrological conditions and applied to predict future conditions under current management practices (baseline model) and for four alternative water management scenarios. These scenarios were developed to predict how lining canals to eliminate seepage losses and concurrently reducing irrigation diversions from the WWR will impact stream flows and groundwater storage with varying levels of MAR. Model results predict that seasonal low flows in the WWR at the downstream reference location will increase an average of 0.13 m(3)/s relative to baseline conditions due to instream water savings with conversion of unlined canals to pipelines (Current MAR-Piped). With MAR increased to 18.0 and 29.9 Mm(3)/year and an additional 58 km piping (Increased MAR-Piped and Maximum MAR-Piped scenarios), the predicted flow increases in the WWR-averaged 0.16 and 0.26 m(3)/s, respectively. Without MAR (No MAR-Piped), flow is predicted to decrease for the months of August and September relative to baseline conditions. The No MAR-Piped and Current MAR-Piped scenarios are predicted to reduce groundwater storage relative to the baseline model due to reduced canal seepage. The Maximum MAR-Piped scenario is predicted to yield groundwater storage that is greater than baseline conditions, while groundwater storage is predicted to be similar to baseline conditions in the Increased MAR-Piped scenario. Model results indicate that canal piping in combination with increased MAR can allow for increased summer flows in the WWR while stabilizing groundwater storage levels for agricultural use and ecological benefits; whereas lining canals without MAR would be detrimental to environmental flows in the WWR and its tributaries.