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Central Valley staff outline CV‑SWAT modeling and township targets to inform groundwater protection
Summary
Central Valley Water Board staff described how grower‑reported INMP/INMP datasets and CV‑SWAT/PSAT modeling produce township loading estimates that underlie groundwater protection values and five‑year reassessable targets, and explained outreach and interim drinking‑water measures in priority areas.
Central Valley Regional Water Quality Control Board staff told the State Water Resources Control Board’s agricultural expert panel on Nov. 14 that farmer‑reported irrigation and nitrogen management reports, combined with a Central Valley adaptation of the SWAT model (CV‑SWAT) and a steady‑state flow tool (PSAT), are the foundation for township‑scale loading estimates and locally tailored groundwater protection targets.
"We run the model for 38 years ... and then we take the average of the last 30 years," said Ken Miller of Formation Environmental, describing the team’s approach to smoothing year‑to‑year variability in nitrate leaching estimates. Eric Warren, program manager for the Central Valley Water Board’s Irrigated Lands Regulatory Program, said the model outputs remain "fundamentally tied to the grower summary report data," which provides a real‑world basis for each field’s leaching estimate.
Presenters summarized how the data flow works: growers submit five‑year farm evaluations and annual irrigation and nitrogen management plan (INMP) summaries; coalitions QA/QC those submissions and file anonymized, field‑level datasets with the regional board; model runs drawn from a "root‑zone library" are matched to parcels and aggregated to the township scale to calculate values and, if necessary, derive targets.
Warren said the region’s 2022 loading estimates show many townships in a roughly 30–60 pound‑per‑acre range and noted that groundwater protection targets approved in 2023 are planned to be reassessed on a five‑year cycle to incorporate new information. Miller and colleagues described a 16‑item uncertainty‑validation workplan — a condition of the targets’ approval — whose results will feed the 2028 update.
Tess Dunham explained how values and targets are implemented through the Irrigated Lands Regulatory Program. For defined priority areas, the nitrate control program ties milestones to compliance timelines so growers meeting milestones can obtain additional time before meeting nitrate water‑quality objectives. The region also conducts domestic‑well testing and supplies interim replacement drinking water via coalition‑operated deliveries and seven public fill stations while longer‑term solutions are developed.
Panelists pressed staff on data access and interpretation. Chair Daniel Geisler asked whether acreage is reported at the field scale; Warren said coalitions retain acreage identifiers but Water Board staff can request parcel‑level information when follow‑up is necessary. Panelists also asked how management‑practice items on the INMP are represented in modeling; Miller said those practices are not modeled as discrete alternate scenarios but the modeling uses applied and removed nitrogen together with reasonable assumptions about practices to estimate leaching.
Presenters emphasized the complementary roles of simple, reportable metrics (such as applied minus removed) and process‑based models like CV‑SWAT: A‑minus‑R provides a grower‑friendly summary metric, while CV‑SWAT/PSAT helps translate loads into groundwater outcomes and accommodate regional hydrology and recharge differences.
No formal actions were taken. Staff said slides and data tables shown at the meeting will be provided to panelists and that work to validate and refine the targets will continue ahead of the 2028 reassessment.
