Panelists for the State Water Resources Control Board's statewide agricultural expert panel spent much of the Dec. 12 meeting debating how regulators should measure and limit nitrogen application on irrigated lands.

Chair Daniel Geisler opened the discussion of Question 6 by asking whether some regions should use a fertilizer-only application limit as a blunt signal to growers or rely on the A minus R (applied nitrogen minus nitrogen removed by crops or other processes) metric that accounts for irrigation water and organic amendments. Several panel members said A minus R is the more direct measure of potential nitrate discharge to groundwater, but they also recognized practical limits on data collection.

Michael Khan said fertilizer-only rates risk duplicating what an A minus R target would capture and could be overly prescriptive for single fields: "For many crops, we don't even know what's the appropriate amount" and growers need flexibility, he said. Hannah Waterhouse and Richard Smith argued fertilizer-percentile signaling (e.g., 80th/90th percentile) can be a pragmatic outreach tool for identifying very high appliers and prioritizing technical assistance in regions lacking robust A minus R data.

Thomas Harder presented APN-level Central Valley analyses averaging A and R across multiple years and showed a tight relationship between applied N and A minus R for many crops. He recommended A minus R as the overarching metric but said A (applied) can provide a simple flag for unusually high, repeated fertilizer use.

Panelists repeatedly noted that where A minus R is not yet calculable because of missing removal coefficients, regions may choose temporary or complementary fertilizer-based signals while investing in the data systems needed to calculate A minus R. Multiple speakers also urged that any fertilizer-only approach be used cautiously and paired with steps to build crop removal coefficients and reporting capacity before creating binding limits.

The panel tasked a drafting team (Daniel Geisler and Michael Khan) with preparing a first draft response to Question 6 that captures this range of views for later circulation.

Next steps: the panel urged regions to prioritize building the data (crop removal coefficients, grower- or parcel-level acreage data) needed to operationalize A minus R and to use fertilizer-only percentiles only as short-term signals where necessary.