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Expert panel urges A‑R metric, three‑year averaging and regionally tailored, iterative limits to curb nitrate pollution
Summary
A state expert panel recommended the A‑R (applied minus removed) mass‑balance metric and three‑year averaging as the preferred approach for regulating nitrogen on irrigated lands, and urged an iterative, region‑specific adoption process with education, targeted assistance and workshops to implement limits where data support them.
A state expert panel convened by the State Water Resources Control Board recommended using the A‑R (applied minus removed) mass‑balance approach—calculated as nitrogen applied to a field minus nitrogen removed in harvested product—as the primary metric for regulating nitrogen on irrigated lands, and advised averaging that metric over three years to reduce volatility in year‑to‑year measurements.
The panel’s president, Dr. Daniel Geisler (UC Davis extension specialist), told the board that A‑R provides a clearer estimate of the quantity of nitrogen available to leach to groundwater. “A and R are estimates, not exact measures,” Geisler said, and recommended a three‑year average to reduce the influence of a single anomalous season. He added that A‑R is preferable to the A/R ratio because the same A/R value can reflect very different absolute application rates and thus different leaching risks.
The panel also recommended a regionally tailored, iterative implementation pathway rather than a single statewide numeric limit. That pathway includes: (1) using existing data to identify regions with sufficient information to begin setting temporary numeric thresholds; (2) deploying education and technical assistance (including soil nitrate testing) to help growers adjust practices; and (3) re‑evaluating limits after initial implementation steps. The panel described modeling tools as useful supplements but not mandatory requirements; models vary by region and can be costly and opaque to calibrate.
On treatment of irrigation‑water nitrogen, the panel said plant‑available nitrogen in irrigation water should be counted in applied nitrogen and suggested an illustrative initial benchmark of 200 pounds of nitrogen per acre for irrigation water in early implementation phases to allow producers time to adapt.
Panelists emphasized that some operation types—small diversified farms, nurseries and many greenhouse systems—lack validated coefficients for “removed” nitrogen and therefore need alternative reporting paths or reduced‑risk categories. The panel endorsed discounts or factor adjustments for organic amendments (for example, applying a C:N–based availability fraction) so that compost‑based and organic systems are not unfairly penalized for slowly released nitrogen.
State staff (Karen Mogis) explained staff is categorizing panel recommendations into three implementation buckets: immediate administrative improvements (data standardization), informational orders or guidance, and formal regulatory changes. Staff and board members discussed next steps including regional workshops with coalitions and third‑party aggregators to improve data quality and develop practicable regional limits. The panel report was approved by the panel on June 5 and staff expects formatted report materials to be posted publicly in July 2026.
No formal regulatory vote was taken at the meeting. Board members directed staff to continue outreach with regional boards, refine data priorities, and return with options and timelines for implementation workgroups and potential regulatory actions.
