ABSTRACT:

PUBLISHED IN WATER RESOURCES RESEARCH 2024
Groundwater return flow to streams is important for maintaining aquatic habitat and providing water to downstream users, particularly in irrigated watersheds experiencing water scarcity. However, in many agricultural regions, increased irrigation efficiency has reduced return flows and their subsequent in-stream benefits. Agricultural managed aquifer recharge (Ag-MAR)—where artificial recharge is conducted via irrigation canals and agricultural fields—may be a tool to recover these return flows, but implementation is challenged by water supply and water management. Using climate-driven streamflow simulations, an integrated operations-hydrology model, and a regional groundwater model, we investigated the potential for Ag-MAR to recover return flows in the Henrys Fork Snake River, Idaho (USA). We simulated potential Ag-MAR operations for water years 2023–2052, accounting for both future water supply conditions and local water management rules. We determined that Ag-MAR operations reduced springtime peak flow at the watershed outlet by 10–14% after accounting for return flows. Recharge contribution to streamflow peaked in July and November, increasing July–August streamflow by 6–14% and November–March streamflow by 9–14%. Furthermore, sites where Ag-MAR was conducted incidental to flood irrigation had more water available for recharge, compared to sites requiring recharge rights, which are junior in priority to agricultural rights. Mean annual recharge volume for the incidental recharge sites averaged 12% of annual natural streamflow, ranged from 269–335 Mm3, and was largely available in April and October. We demonstrate Ag-MAR can effectively recover groundwater return flows when applied as flood irrigation on agricultural land with senior-priority water rights.

ABSTRACT:

This is a repository for the code + data associated with the publication titled "Assessing downstream aquatic habitat availability relative to headwater reservoir management in the Henrys Fork Snake River" by Christina N Morrisett, Robert W Van Kirk, and Sarah E Null in River Research and Applications (2023).

Reservoirs are sometimes managed to meet agricultural and other water demands, while also maintaining streamflow for aquatic species and ecosystems. In the Henrys Fork Snake River, Idaho (USA), irrigation-season management of a headwater reservoir is informed by a flow target in a management reach ~95 km downstream. The target is in place to meet irrigation demand and maintain aquatic habitat within the 11.4 km management reach and has undergone four flow target assignments from 1978 to 2021. Recent changes to irrigation-season management to maximize reservoir carryover warranted investigation into the flow target assignment. Thus, we created a streamflow-habitat model using hydraulic measurements, habitat unit mapping, and published habitat suitability criteria for Brown Trout (Salmo trutta), Rainbow Trout (Oncorhynchus mykiss), and Mountain Whitefish (Prosopium williamsoni). We used model output to compare habitat availability across two management regimes (1978–2017 and 2018–2021). We found that efforts to minimize reservoir releases in 2018–2021 did not reduce mean irrigation-season fish habitat relative to natural flow, but did reduce overall fish habitat variability during the irrigation season compared to streamflow management in 1978–2017. Field observations for this research led to an adjusted flow target in 2020 that moved the target location downstream of intervening irrigation diversions. Using our model output, we demonstrated that moving the location of the target to account for local irrigation diversions will contribute to more consistently suitable fish habitat in the reach. Our study demonstrates the importance of site selection for establishing environmental flow targets.