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A “water budget” is the accounting of the total groundwater and surface water entering and leaving a basin, including the changes in the amount of groundwater stored in the basin. Basic components of water budgets are:
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Community water systems, such as those serving municipalities, are required by state law to monitor the quality of the water they supply to the public. Small water systems, such as schools, mobile home parks, and food facilities that use groundwater wells to supply their drinking water are also required by the state to monitor water quality. Other state programs require groundwater quality monitoring for locations near regulated facilities (an example of a type of regulated facility includes leaking underground fuel tanks). A statewide database through the State Water Resources Control Board Groundwater Ambient Monitoring and Assessment Program (GAMA) integrates and displays groundwater quality information.
In general, there are linkages between surface water and groundwater. Understanding the spatial and temporal interconnection between surface water and groundwater is a requirement of the GSP and appropriate management actions have been included in the Plan.
Irrigation system efficiency is a ratio of the amount of irrigation water consumed by plants to the total amount of irrigation water supply. The irrigation efficiency depends on a variety of factors, including topography, soil characteristics, water conveyance system, crop type, and irrigation practices.
The Napa River is designated under the Clean Water Act as impaired by nutrients, pathogens, and sediment. As of October 2020, the State Water Resources Control Board and San Francisco Bay Regional Water Quality Control Board have approved delisting non-tidal areas of the Napa River for nutrient impairment.
The Groundwater Sustainability Plan Advisory Committee (GSPAC) received a presentation from Paul Wells of the Department of Water Resources (DWR) on November 12, 2020 regarding SGMA requirements for addressing the effects of climate change in GSPs. Technical staff working on the GSP have separately coordinated with Tyler Hatch (a DWR Supervising Engineer and groundwater modeler) and Paul Wells in developing the approach to incorporating climate change into the Napa Valley Integrated Hydrologic Model and GSP for the Napa Valley Subbasin.
Given uncertainty in future climate projections, climate change was evaluated using multiple future climate scenarios. The analysis relied on downscaled results from peer-reviewed global atmospheric circulation models to generate multiple sets of climactic (e.g. precipitation and potential evapotranspiration) and hydrologic (e.g. stream inflow) inputs for Napa Valley. The projections spanned a 50-year period, consistent with the requirements of the GSP Regulations. The future scenarios used in the model have been selected because they provide meaningful bounds on best-available climate projections for the North Bay. The future scenarios were chosen based on collaboration with the Pepperwood Preserve and U.S. Geological Survey as described in the presentation by Lisa Micheli at the November 12, 2020 GSPAC meeting.
The results of the climate change analysis are included as part of the projected water budget presented in section 8 of the GSP.
Surface waters that support groundwater dependent ecosystems (GDEs) in the Subbasin include springs, wetlands, and surface water channels. These include springs in the vicinity of Calistoga and estuarine and riverine tidal channels in the southern portion of the Subbasin, extending to within the City of Napa.
The amount of water used by groundwater dependent ecosystems (flowing from the aquifer into the Napa River) is estimated at 4,184 acre-feet.
Overall groundwater levels in the main Napa Valley Subbasin have been stable for decades. Groundwater conditions outside the Napa Valley Subbasin are more variable, such as in the Milliken-Sarco-Tulucay (MST) area. In addition to the effects of the recent drought, the productivity of an individual well can depend on a number of things including the depth and serviceable life of the well, local aquifer properties, and amount and rate of nearby pumping from surrounding wells. Report dry wells here.
Generally, groundwater levels across the Napa Valley Subbasin have been stable for decades. In limited areas, newer wells may be deeper to produce at dependable rates. This would include areas where seasonal variability is high, or the Northeastern Napa Subarea where water level declines in wells monitored by the County east of the Napa River were observed over approximately the decade prior to 2009, but have since stabilized.
Reaches of the Napa River have over many decades (since the 1930s) experienced low to no-flow conditions during the summer-to-fall period for a variety of reasons. Stream flow is very dependent on seasonal rainfall, small dams (both legal and illegal) that have been constructed to block stream flow, withdrawals of surface water (both legal and illegal) from the creeks, as well as reduced groundwater discharge into the stream channel. The duration of annual no flow days varies from year-to-year and increases during extended droughts as during recent years.
The Basin Analysis Report finds that overall, groundwater levels in the Napa Valley Subbasin have been stable for decades, demonstrating that current groundwater pumping has not contributed to chronic depletions of groundwater storage and that pumping has likely been below the sustainable yield for the Subbasin. Surface water and groundwater are connected; therefore, seasonal and year to year variability in precipitation and other factors have affected both surface water and groundwater. Since at least the 1930s, periods of no flow days have been observed in the Napa River system, particularly during drier years. Based on the analyses of surface water and groundwater interconnections, including the relationship of this connection to seasonal and annual groundwater elevation fluctuations, the Basin Analysis Report uses 16 wells (and other data including stream gage data) in the Subbasin to monitor groundwater level impact on the Napa River. As long as the fall water levels in these 16 wells remains above the determined level, (the “minimum threshold”), the contribution of groundwater to flow in the Napa River is determined to be no less than has occurred historically in the fall. On average, it is preferable for fall water levels in these wells to approximate their individual measureable objective, which is a level higher than the minimum threshold.
Water levels in northeastern Napa Subarea wells monitored by the County east of the Napa River have stabilized since 2009, though declines were observed over approximately the prior decade. The study, conducted between 2016 and 2017, included evaluation of the potential effects from pumping in the overall Study Area, potential mutual well interference in an area of interest near Petra Drive, and potential streamflow effects. As part of the Special Study, a transient numerical groundwater flow model was developed that incorporates the data collected for a base period of water years from 1988 to 2015 to analyze groundwater conditions in the Study Area and the area of interest near Petra Drive. At their meeting on October 24, 2017, the Board of Supervisors supported the findings and recommendations of the Special Study Report and directed staff to develop documentation (2018 Amendment) to formally establish the Northeast Napa Management Area covering approximately 1,960 acres within the 45,928- acre Napa Valley Subbasin. This 2018 Amendment provides additional detail about conditions in the Northeast Napa Management Area and establishes additional sustainable management criteria and management actions intended to support continued groundwater sustainability in the Napa Valley Subbasin as a whole. The 2018 Amendment designates seven representative monitoring sites as a subset of the monitoring sites in the Northeast Napa Management Area for the purpose of monitoring groundwater conditions that are representative of the management area, consistent with the GSP Regulations (Section 354.36). For SGMA purposes for the Napa Valley Subbasin, these seven sites are where sustainability indicators are monitored, and minimum thresholds and measurable objectives are defined. For more information, read the Northeast Napa Special Groundwater Study and Presentation.