Validation of the Impacts of Recent Aquifer Management on the Eastern Snake Plain Aquifer in Idaho, <scp>USA</scp>

An ongoing major challenge faced in portions of the western United States is to stop the decline of aquifers that are hydraulically connected to rivers. As these aquifers decline, streamflow is depleted, resulting in impacts to agriculture, environmental flows, and hydropower production. In 2014, the Idaho Water Resource Board initiated an aquifer recharge program, and in 2015 a historic settlement agreement (hereafter referred to as the Settlement Agreement) was signed by surface water users with senior water rights and groundwater pumpers with junior water rights to stop the decline of the eastern Snake Plain Aquifer (ESPA) in southern Idaho (SWC-IGWA 2015). Here, we assess mitigation measures they have undertaken to reverse the downward trajectory of groundwater levels in the ESPA using drought indices correlated to the combined head change of a suite of groundwater monitoring wells. The results were then compared against the predictions of the Enhanced Snake Plain Aquifer Model (ESPAM), which is a MODFLOW-based aquifer model. The drought indices indicate that without the aquifer recharge program and reductions in groundwater pumping, the aquifer head would have been 1.1 to 1.3 m lower than observed in 2023, indicating implemented water management practices reduced the volumetric loss to the aquifer by 2500 million cubic meters (2,000,000 acre-feet). The result, therefore, implies that Idaho water users and managers have succeeded in changing the trajectory of ESPA water levels.