State Water Board expert panel debates crop-specific nitrogen limits; panelists favor regionally tailored thresholds and multi‑year averages

Members of the State Water Resources Control Board's statewide agricultural expert panel working group spent most of a working meeting debating whether groundwater‑protective nitrogen thresholds should be set on a crop‑by‑crop basis or instead by region and aggregated source area.

Panel chair Daniel Geisler, Cooperative Extension specialist in Nutrient Management at UC Davis, opened the discussion by asking whether the data presented so far support crop‑specific limits and what additional information the panel should request. "We should probably have a brief look or an opportunity to talk about our general impression of the data," Geisler said as the group moved into the charge questions.

Why it matters: any numeric limit will affect how growers measure fertilizer use and reporting, and, if tied to enforcement, could trigger costly penalties. The panel repeatedly returned to the central public‑health benchmark of 10 milligrams per liter nitrate (the drinking‑water standard) and the practical question of what loading on the landscape translates to that standard in groundwater.

Most panelists said crop‑specific, single‑season limits would be difficult to justify statewide because of regional differences in crops, cropping rotations (single, double or triple crops), irrigation practices, and hydrology. Thomas Harder, a hydrologist with UC ANR, urged the group to think in terms of groundwater source areas and travel time: "What we really want in this well is this mixture to be below the drinking water limit," Harder said, explaining that water captured by a well is mixed across space and time and therefore that management choices should account for that mixing.

Practical alternatives discussed included: 1) a regional or landscape-level mass balance tied to the groundwater capture area for impacted wells; 2) multi‑year rolling averages to smooth crop‑rotation effects; and 3) percentile‑based thresholds that flag the highest‑emitting fields for outreach and intervention (for example, a proposal to focus on the worst 20 percent of fields and encourage them to adopt practices used by the better 80 percent). Daniel Geisler illustrated how an 80th‑percentile approach could be used in a crop cohort: for one silage‑corn dataset he cited, the 80th percentile equaled about 58.4 pounds of nitrogen per acre in 2023.

Panelists emphasized practices and measurements that reduce uncertainty and help growers manage nitrogen proactively: in‑season nitrate testing, nitrogen removal (R) measurements from crop yields, improved irrigation management, cover crops, and high‑carbon soil amendments. Richard Smith, a retired farm advisor who has worked on Central Coast vegetable crops, and Michael Khan, UC Cooperative Extension irrigation and water resources farm advisor in Salinas, both highlighted the importance of regionally developed crop coefficients and removal coefficients to convert grower yields into nitrogen‑removed figures.

Several panelists noted that some regions already have robust datasets (Central Valley coalitions, Central Coast studies) that can support regional targets while other regions (nurseries, diverse specialty farms, some southern California areas and deserts) lack sufficient data to set crop‑specific numeric limits.

Public comment and external input: public commenters raised enforcement and data gaps. Tess Dunham (public comment) quoted a court of appeal describing the tradeoff regulators face between a reliable food supply and protecting state waters. Zoe Robertson (Stanford Law student) told the panel that her review of 2023 confined animal feeding operation (CAFO/CAFO‑style) annual reports in the Central Valley found large quantities of nitrogen unaccounted for in many reports, which could mean unreported leaching; she urged the panel to consider agricultural animal sources.

What the panel will do next: the group agreed to continue question 1 and related questions at the October 22 working meeting and to request additional targeted data and speakers (see companion article on speakers and data requests). Panelists also repeatedly recommended using multi‑year summaries or landscape‑level mass balances rather than a single crop, single‑season limit for regions with complex rotations.

Ending: Panelists framed any final numeric recommendations as part of a technical package to inform the State Water Resources Control Board's policy decisions. Sam (staff) reminded the group that the panel's charge is technical and that the board itself will weigh economic and policy tradeoffs when it considers regulatory design.