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Napa Valley Subbasin is categorized by DWR as one of 46 high priority groundwater basins statewide. Medium and high priority basins are subject to the Sustainable Groundwater Management Act (SGMA) requirements. The Department of Water Resources (DWR) prioritized all basins in the state according to the following criteria:
The Napa Valley Subbasin is categorized as high priority primarily due to the amount of irrigated lands, the density of wells, projected population growth, and the degree to which people rely on groundwater in the Subbasin.
DWR’s basin prioritization process is not a determination of whether groundwater basins are being managed sustainably. Rather, it is a way for DWR to determine the reliance on groundwater in individual basins across California and whether those basins should be subject to the requirements of SGMA.
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The Sustainable Groundwater Management Act (SGMA) defines sustainable groundwater management as managing and using groundwater without causing undesirable results such as: chronic groundwater level declines, groundwater storage reductions, seawater intrusion, water quality degradation, land subsidence, and surface water depletions. SGMA requires that groundwater basins classified as either medium or high priority by the California Department of Water Resources achieve sustainable groundwater management within 20 years of adopting a Groundwater Sustainability Plan.
SGMA defines undesirable results as “one or more of the following effects caused by groundwater conditions occurring throughout the basin:
GSPs must address undesirable results and undertake management actions to ensure that they are avoided within 20 years of GSP adoption.
As part of SGMA, DWR recognizes 5 groundwater basins and subbasins in Napa County. Here is the list of the basins and subbasins, along with the priority that DWR gave to each basin:
High and medium priority basins are subject to a variety of rules and regulations under SGMA. Low and very low priority basins are not required to take any action at this time. Learn more about requirements for each priority level here.
The Sustainable Groundwater Management Act (SGMA) requires that Groundwater Sustainability Plans (GSPs) or Alternatives to a GSP be developed for medium and high priority groundwater basins as delineated and ranked by the State Department of Water Resources (DWR). The hillsides do not fall within the Napa Valley Subbasin that the Basin Analysis Report addresses. Because the hillsides do not act as a basin, but instead as thousands of discrete subareas based on local geography, it is not scientifically or economically practical to “study the hillsides”. However, the hillsides are included in the Napa Valley Subbasin water budget by incorporating uplands runoff and subsurface inflow.
The Sustainable Groundwater Management Act requires that Groundwater Sustainability Plans (GSPs) or Alternatives to a GSP be developed for medium and high priority groundwater basins as delineated and ranked by the State Department of Water Resources (DWR). The MST and Carneros Subareas are not state-defined basins, but they are subareas that Napa County has established based on watershed boundaries and the County’s environmental resource planning areas for the purposes of local planning, understanding, and studies. With regards to the MST, it is one of the most monitored areas of the county, with data dating back many decades. There are significant land use controls in place in the area (the county has not approved a discretionary project in the MST that couldn’t meet the “no net increase” standard since 2004), and significant effort has gone into constructing a recycled water pipeline to the area, that became operational in 2016. The Carneros Subarea partly overlaps with the Napa Sonoma Lowlands Subbasin which is a DWR-designated very low priority Subbasin for which a GSP or Alternative is not required. Updates on groundwater conditions in the MST and Carneros Subareas have been and will continue to be included in the County’s Annual Groundwater Monitoring Reports.