Groundwater relationships to pumping, precipitation and geology in high-elevation basin, Sierra Valley, CA

Sierra Valley, located in the northern Sierra Nevada, California, serves as the Middle Fork Feather River headwaters and provides surface water to Oroville Dam of the California State Water Project (SWP). Under California’s Sustainable Groundwater Management Act (SGMA), the Sierra Valley sub-basin has been designated a medium-priority basin, due to chronic groundwater declines and the valley’s high ecological value as the largest freshwater marsh and meadow system in the Sierra Nevada. The Sierra Valley Groundwater Management District (SVGMD) serves as the Groundwater Sustainability Agency (GSA) for the Sierra Valley sub-basin. As such, SVGMD is tasked through SMGA with achieving sustainable groundwater management over an approximate 20-y timeframe. The first step is the development of a Groundwater Sustainability Plan (GSP) (to be completed by January 2022) that 1) hydrologically assesses the basin, 2) identifies methods and protocols to track groundwater trends, and 3) develops an initial suite of actions to move the basin towards groundwater sustainability. ... Our investigation builds on previous watershed studies and further establishes the Sierra Valley watershed as a highly complex hydrologic system. These complexities include: large variation in precipitation phase and quantity throughout the watershed; geologic features that restrict both vertical and lateral groundwater flow; many water inflow pathways, both surface and sub-surface, that are logistically impossible to quantify by conventional monitoring means. Prior attempts at developing accurate water budgets and numerical models of the watershed have been hindered by the uncertainty these factors present. Thus, though a hydrologic budget is required by SGMA for the development of the GSP, numerical models will be of limited utility as either tools to derive hydrologic budgets or to help determine the efficacy management actions to achieve sustainable groundwater conditions. In developing strategies to address undesirable groundwater conditions, we recommend an adaptive management approach paired with targeted and defensible data collection with standardized data collection, management and quality control procedures