Assessment of Potential Gila River Flow Alteration on Riparian Groundwater Conditions

Groundwater modeling was conducted to assess effects of potential diversions from the Gila River in New Mexico, pursuant to the authority and constraints of the Arizona Water Settlements Act (AWSA) of 2004, on riparian groundwater conditions. Among concerns were the effects of flow alteration on riparian vegetation in the Cliff-Gila Valley. Defining the relationship between the hydrograph, existing and altered, and groundwater is a critical step in characterizing the flow-ecology relationship for groundwater-dependent riparian vegetation under the wide range of existing flow conditions and with potential diversions. The assessment is streamlined through focus on periods and conditions of largest potential change identified through flow alteration statistics using The Nature Conservancy’s Indicators of Hydrologic Alteration (IHA) software, and examination of effects over periods associated with vegetation recruitment and early growth. Framed accordingly, hypothetical flow alteration scenarios are simulated with a high-resolution groundwater model incorporating a boundary condition supporting flow-dependent stream stage and width. Simulated groundwater depths and changes under baseline and altered hydrographs illustrate effects of potential diversions on groundwater within the topographically diverse riparian zone and provide a foundation for evaluation of questions regarding implications for groundwater-dependent riparian vegetation.