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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.
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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 will be included in the Plan.
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:
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 will be evaluated using multiple future climate scenarios. The analysis will rely 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 will span a 50-year period, consistent with the requirements of the GSP Regulations. The future scenarios that will be 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 will be included as part of the projected water budget presented in the second draft of GSP Section 8 planned for release in Summer 2021.