TO: Technical Advisory Group for the Napa County Groundwater Sustainability Agency
FROM: Brian D. Bordona, Director of Planning, Building and Environmental Services
REPORT BY: Jamison Crosby, Natural Resources Conservation Manager
SUBJECT: State Water Board: Water Supply and Demand Assessment for the Napa River Watershed
RECOMMENDATION
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Receive a presentation from staff of the State Water Resources Control Board (SWRCB) Supply and Demand Assessment Unit and Paradigm Environmental, Inc. on Modeling Water Supply and Demand in the Napa River Watershed.
Procedure
Staff introduces the item
Presentation
Questions and answers from the TAG
Public comments
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BACKGROUND AND DISCUSSION
The State Water Resources Control Board (State Water Board) is responsible for allocating surface water through California’s water rights priority system. Watershed-specific supply information from year to year is generally lacking, making water management planning difficult, especially in times of water shortage. Recent droughts (2013-2016, 2021-2023) highlighted the need for specialized data and tools to assess water availability and water demand and to evaluate how limited water resources are allocated. The State Water Board is initiating a new effort that will use an approach similar to the one used for the Russian River during the most recent drought. It will enable the State Water Board to prepare for future dry conditions in other watersheds and provide data to help local water managers better understand supply constraints, develop local responses and plan for droughts.
In 2021, in response to the drought emergency in Sonoma and Mendocino Counties, the State Water Board developed tools and information to better understand water supply and demand in the Russian River watershed. Staff developed a water supply model and evaluated water diversion data to estimate surface water demands. Staff also used a tool to allocate water to right holders based on the modeled available surface water supply, water demand data and water right priorities. The water supply and demand data, when integrated into the water allocation tool, was a key component in the State Water Board’s implementation of drought emergency actions in the watershed.
Evaluating water supply and demand in a watershed enables improved comparison of the available water in the system and the demands associated with different water uses. State Water Board staff are working with Paradigm Environmental, Inc. to develop water supply (hydrologic) models that assess surface water availability in select watersheds where low flows and drought conditions may threaten water supplies, impair critical habitat, and create uncertainty for water users. Water supply modeling tasks are underway in Butte Creek (tributary to the Sacramento River), the Napa River, the Navarro River, and six additional watersheds. These watersheds were selected because: 1) water management can benefit from such a modeling framework; and 2) they contain areas of salmonoid habitat and important fisheries, especially areas with flow-related concerns. All data and tools developed under this effort will be open source and available to the public so local water managers and other interested parties can assess surface water availability conditions and evaluate potential management options.
The Napa River is one of the major tributaries to the San Pablo and San Francisco Bays. The Napa River watershed is part of the San Pablo Bay drainage area, which shares a boundary with the Russian River watershed to the west, Upper Putah Creek watershed to the north-east, and Suisun Creek watershed to the east. The non-estuarine watershed area drains approximately 283 square miles.
The watershed represents an important habitat for native aquatic species and spawning ground for anadromous fish, especially chinook salmon and steelhead trout. However, there have been substantial declines in salmonid populations over time; coho were once present but extirpated in the late 1960s. The decline in anadromous fish populations within the Napa River watershed was linked to an increase in sediment delivery and other factors including stream temperatures above that which supports salmonid life and low dry season flows. These factors led to the development of a Total Maximum Daily Load (TMDL) for sediment in 2009 and implementation of the Napa River and Sonoma Creek Vineyard General Permit Program to control erosion and manage stormwater runoff.
The Loading Simulation Program in the programming language C++ (LSPC) is used to estimate surface water availability (water supply) for this watershed. LSPC has been used extensively in California to model and manage the state’s unique watersheds. The model uses a large library of publicly available data from state and federal agencies, including the following types: 1) Meteorological: Historical weather data such as precipitation, evapotranspiration, air temperature, vapor pressure, and wind speed allow for modeling conditions that affect water supply; 2) Hydrological: Current and historical data on the river’s streamflow rate is used for calibration and validation of the model; 3) Water use and diversion: Data showing how much water has been taken out of the river in the past serves as a proxy for watershed demand and how it affects streamflow; and 4) Geospatial: Data that describe where the boundaries and channels of the watershed are, as well as its physical properties, such as soil type, land cover type, and topography. These datasets are important for understanding characteristics that influence how water moves through and is absorbed by the landscape.
All datasets used in the model are subject to extensive quality control and quality assurance procedures that ensure their accuracy. This is particularly important when it comes to incorporating required water use and diversion data that is self-reported by water right holders. The cleaned data is then used to run the water supply model to simulate hydrological processes in the watershed at the scale of small tributaries (small catchments), allowing for water management decisions to be made at a very fine geographic scale.
San Francisco Estuary Institute (SFEI) developed a LSPC model over nine bay area watersheds including Napa River. The LSPC model of the Napa River for this work was built off the model SFEI developed with adjustments to align the model with these work objectives. These adjustments include: 1) subdividing the watershed into small catchments to allow water allocation at the scale of small tributaries; 2) further model calibration to better capture low flows; and 3) further constraining the model using remotely sensed evapotranspiration data.
STATE BOARD AND NAPA COUNTY COORDINATION
The State Water Board first introduced Napa County to the Supply and Demand Assessment (SDA) program at an initial virtual meeting with SDA staff on November 14, 2023, where SDA staff described the Napa River Watershed as being selected as a Pilot Watershed. On February 20, 2024 SDA staff, Napa County, and the County’s technical consultants met to discuss the Napa Valley Integrated Hydrologic Model (NVIHM) developed for the County as an integral part of the Napa Valley Subbasin Groundwater Sustainability Plan (GSP). Prior to the February 20th virtual meeting, on February 14th, Napa County provided both a link to a recording with background information about the NVIHM model and a presentation with additional information about future model-related efforts, including ongoing refinement of surface water and groundwater interaction and the effects of drought and pumping on variability in surface water flows. The State Water Board held three public meetings on May 14, May 21, and June 3, 2024. Members of the Napa County technical team attended the June 3rd public meeting (link to the meeting recording can be found here: <https://www.youtube.com/watch?v=BTIGsbRRS6Y>) and noted multiple concerns related to groundwater interactions within the selected watersheds.
The NVIHM covers the entire Napa River Watershed. The Napa-Sonoma Valley Groundwater Basin portion of the Watershed (and also a buffer surrounding the basin boundary where vineyard land use has expanded up the hillside) is based on the MODFLOW-OWHM platform. The NVIHM is calibrated to streamflows and groundwater elevations in wells with long historical monitoring records. The interconnection between surface water and groundwater in the Napa Valley Subbasin has been recognized for decades and many efforts have supported extensive hydrogeologic conceptualization, including hydrogeologic parameters and fault structures that have been represented in the NVIHM. Napa County has engaged with the academic and grower communities regarding irrigation practices, including understanding of some dry farming, tailwater returns, and use of onsite storage ponds for rainwater and temporary storage of permitted surface water diversions for later seasonal irrigation.
Based on Napa County’s initial understanding of the SDA unit’s objectives to develop supply models and demand assessments, especially to guide management actions during future dry conditions, the County supports the overall objectives of the SDA unit’s program. However, close coordination is needed between the State Water Board and Napa County to ensure that overall modeling efforts accurately assess water availability and critical low flow issues. Following (or in coordination with) development of the LSPC model for the Napa River watershed, Napa County believes it is critical that the SDA unit consider next steps to utilize a fully integrated surface water-groundwater flow model such as the NVIHM in order to address the following factors:
• Low flows in the Napa River Watershed are groundwater-supported; focusing solely on surface water diversions will be counterproductive to maintaining low-flow conditions.
• The largest impact to flows is groundwater extraction; any evaluation of low-flow scenarios needs to assess both groundwater and surface water management, including the use of and temporal variables associated with onsite storage of rainwater and permitted surface water diversions.
• Tools have already been developed (NVIHM) and are actively maintained to evaluate streamflow within the Napa River Watershed.
• The LSPC is best suited to modeling surface water hydraulics in the upper Napa River Watershed; the LSPC model is a very simplified representation of groundwater pumping, which cannot adequately represent surface water and groundwater interactions.
It is essential that the State Water Board and Napa County coordinate activities and outreach during the SDA’s preliminary LSPC model development and subsequent efforts to use a modeling platform that fully integrates surface water and groundwater connectivity within the Napa Valley Subbasin. The County has many programs currently underway to enhance countywide stewardship efforts, collect additional data, refine the NVIHM, and implement five GSP implementation workplans (e.g., Napa County Water Conservation, Groundwater Pumping Reduction, and Interconnected Surface Water and Groundwater Dependent Ecosystems Workplans; Stormwater Resources Plan; and Communications and Engagement Plan). For successful community engagement, it is critical that Napa County and the State Water Board collaborate and facilitate stakeholder understanding of the purpose and utility of the County’s and the State Water Board’s programs.
QUESTIONS FOR STATE BOARD AND TAG DISCUSSION
The TAG and technical team may consider posing the following questions to Water Board staff following their presentation:
1. Low-flow, temperature, and sediment have been cited as concerns and support the need for watershed tools to manage flows. What conditions, i.e. constituents, locations and flow criteria, will determine when and where surface water allocations occur? If the criteria for allocations have not yet been set, what is the process that will be used to set the criteria?
2. Napa Valley faces many unique circumstances due to high groundwater levels in many areas of Napa Valley, these include extensive tile drain systems and direct root uptake of groundwater. What other viticulture-specific conditions occur in the Napa River Watershed that should be considered to better represent water use within the State Water Board water model?
3. Has the LSPC modeling framework been applied to areas with significant interactions between surface water and groundwater?
ENVIRONMENTAL IMPACT
ENVIRONMENTAL DETERMINATION: The proposed action is not a project as defined by 14 California Code of Regulations 15378 (State CEQA Guidelines) and therefore CEQA is not applicable.
SUPPORTING DOCUMENTS
A. PowerPoint presentation by SWRCB and Paradigm Environmental