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: Update on Napa Valley Integrated Hydrologic Model updates and streamflow depletion model scenarios
RECOMMENDATION
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Provide an update on streamflow depletion model scenarios and model updates to the Napa Valley Integrated Hydrologic Model (NVIHM).
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Procedure
Staff introduces.
Questions and answers with the TAG.
Public comments.
BACKGROUND AND DISCUSSION
The Napa Valley Integrated Hydrologic Model (NVIHM) was developed by LSCE from 2020 through 2021 to support key elements of the Napa Valley Subbasin Groundwater Sustainability Plan (GSP). NVIHM is a numerical model which simulates landscape, surface water and groundwater processes and interactions using an integrated approach and is an important tool used to explore and understand the complex hydrologic interactions throughout the Napa Valley Subbasin (Subbasin). Originally, the NVIHM was calibrated through Water Year (WY) 2019 and has been updated annually based on climate and reported water use. The NVIHM continues to match observed values in wells with similar fit statistics as the calibrated model. Monthly streamflow at the Napa River Pope Street and Oak Knoll USGS gages also align with observed values.
LSCE is developing updates and refinements to NVIHM to better represent key physical processes and elements in the landscape, streams, groundwater dependent ecosystems, and the groundwater system. As previously presented to the TAG, the total water use as measured by total evapotranspiration (ET), is variable across the Subbasin. Multiple data sources, including local sensors as well as remotely sensed data, are being used to update modeled ET. Data specifically compiled by Napa County, including refined stream geometries, well locations, and land use are being incorporated. As previously identified by the TAG, accounting for soil moisture is a high priority in the Subbasin. LSCE has been in coordination with the United States Geological Survey (USGS) to update the model platform to better account for soil moisture. These updates, as well as the additional calibration data that has been collected, will update and inform the current understanding of water movement through the Subbasin.
NVIHM has been used to estimate historical flows of a system but can also be utilized to learn more about how the system may behave under certain conditions. These exercises are called “scenarios” and involve the development of a set of hypothetical management changes that are executed in the model. The results can be used to estimate the effect of possible changes to the hydrologic system. One scenario that is run every year is the stream depletion scenario, which is utilized in evaluating the Interconnected Surface Water (ISW) Sustainable Management Criteria (SMC). The stream depletion scenario used in the ISW SMC involves simulating a condition where there is zero pumping from all agricultural and outdoor applications. Additional scenarios have been developed and run to evaluate stream depletion in the Calistoga and Saint Helena areas to better assess the impact of sub-regional pumping on stream depletion in the Napa River in the northern portion of the Subbasin. Results of these scenarios will be presented in the TAG meeting.
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. Napa County Groundwater Sustainability Agency, Napa Valley Integrated Hydrologic Model Updates and Scenarios (LSCE, April 2025)