TO: Technical Advisory Group for the Napa County Groundwater Sustainability Agency
FROM: Brian Bordona - Director of Planning, Building and Environmental Services
REPORT BY: Jamison Crosby, Natural Resources Conservation Manager
SUBJECT: Overview and Updates to the Napa Valley Integrated Hydrologic Model
RECOMMENDATION
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The Technical Advisory Group (TAG) members will receive a presentation from Luhdorff and Scalmanini, Consulting Engineers (LSCE) featuring an overview of ongoing updates to the Napa Valley Integrated Hydrologic Model. This will include an overview of key model developments pertaining to the simulation of historical and projected hydrologic conditions and water use. Framing questions will also be provided to receive direction and feedback from the TAG.
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EXECUTIVE SUMMARY
LSCE staff provided several presentations to the Groundwater Sustainability Plan Advisory Committee (GSPAC) during 2021 focused on the development and use of NVIHM in supporting the Groundwater Sustainability Plan (GSP) and sustainable management of groundwater. LSCE has continued to develop and update NVIHM as part of GSP implementation to better refine key elements identified through feedback and comments received from the GSPAC, recommendations in the GSP, and input from the TAG. This presentation introduces the current and planned efforts in updating the representation of surface water and watershed response, agricultural water use and soil moisture, subsurface geology, and climate (Supporting Document A). A presentation is also posted on the County’s website summarizing relevant background information pertaining to NVIHM development (Supporting Document B).
Procedure
Staff introduces.
Questions and answers with the TAG.
Public comments.
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.
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 GSP. NVIHM is a numerical model which simulates landscape, surface water and groundwater processes and interactions using an integrated approach. LSCE provided several presentations to the GSPAC highlighting model development and results during 2021 and provided detailed documentation of NVIHM in the Napa Valley Subbasin GSP.
LSCE is developing updates and refinements to NVIHM to refine the representation of key physical processes and elements in the landscape, streams and groundwater dependent ecosystems, and the groundwater system based on feedback and information provided by the GSPAC, recommendations included in the GSP, input from the TAG, and public and stakeholder comments. NVIHM updates currently underway include:
• Refinement and enhancement of surface water processes. The Basin Characterization Model (BCM) used to represent the upper watershed response is being updated to better reflect local conditions and changes in land use. Additional complexity is being added to streams specified in NVIHM to better capture channel geometry and changes to channel morphology resulting from stream restoration.
• Updates to consumptive use and applied water. Estimates of the timing and amount of crop water use are being refined based on remote sensing and field measurements. LSCE is coordinating with the NVIHM model platform developers and the U.S. Geological Survey to enhance the representation of soil moisture storage and better accounting of on-farm surface water storage. LSCE is also working on updates to better estimate evapotranspiration and how it is represented in the model structure.
• Refinement of physical conceptualization. The representation of subsurface geologic structures and subsurface processes is being continually refined based on new data and information. This includes the refinement of aquifer geometry (thickness, distribution and physical properties of alluvium and volcanic geologic units) and depth and distribution of groundwater pumping wells.
Future updates to NVIHM include:
• Update to model projections. Climate projections used to develop projected water budgets and hydrologic response will be updated to utilize climate scenarios from more recent global circulation models (GCMs) and current best practices.
• Updates to observations used to calibrate and constrain NVIHM. Additional measured or remotely sensed data (e.g., evapotranspiration, Stream Watch observations, interconnected surface water (ISW) monitoring sites) will be added as observations to NVIHM to better evaluate and constrain model results.
FRAMING QUESTIONS
The following framing questions will be asked for the TAG to solicit feedback and direction on updates to NVIHM.
1. What other watershed characteristics could be important to refine or update?
2. Are there any other data that could be leveraged to better represent surface water processes?
3. How can we best leverage available data to develop inputs that are representative of land and water use in the Napa Valley?
4. What are the questions we should be asking growers and other stakeholders to collect the information we need?
SUPPORTING DOCUMENTS
A. Presentation-Hydrologic Model Updates
B. Hydrologic Model Background Presentation -
<https://tinyurl.com/NVIHM-Model-Presentation>