The State Water Resources Control Board’s second statewide agricultural expert panel concluded Nov. 19 that A minus R (applied nitrogen minus nitrogen removed) is an appropriate, practical metric for growers to report and for regulators to compare performance at the field level, while hydrogeological models (including CV SWAT used in the Central Valley) should inform regional targets and long-term groundwater impacts.

Panel chair Daniel Geisler said the group viewed A minus R as accessible to growers and useful for comparing efficiency among similar crops or cropping systems. "A minus R is a valid metric to use, to estimate potential of metric discharge," Geisler said during the meeting, noting the metric’s suitability for field-scale reporting and comparison.

Panelists repeatedly emphasized a complementary role for models. Kenneth Miller, who works with CV SWAT implementations, told the panel CV SWAT integrates soils, climate and INMP inputs and "provides additional context to A minus R and provide an estimate of leaching that considers the full nitrogen cycle and water balance." Miller said coalitions use the model at field (soil-mapping-unit) scales and roll results up to township-level targets.

At the same time, several panelists cautioned against requiring growers to compute process-level losses (volatilization, denitrification) themselves. "The modeling would have to be done by the coalition or some third party and then provided to the grower," Ruth Dahlquist Willard said, arguing growers should report the data they already have while third parties run models that estimate unobserved processes.

Panel members recommended flexibility by region: where robust hydrogeologic or calibrated modeling frameworks exist, coalitions may use models to refine targets and interpret A minus R; where they do not, A minus R remains a useful starting point. The panel also recommended that model-derived factors be used in implementation and target-setting by coalitions or regional boards, not as mandatory individual grower reporting requirements.

The panel requested clearer document language to avoid implying growers must measure processes that models can estimate. It also called for more research to improve removal coefficients and model calibration in regions with diverse crops and limited existing hydrogeologic data.

Next steps: the panel asked staff to compile written comments and revise draft responses for the Dec. 5 meeting and to reflect the distinction between grower reporting metrics and coalition/modeling interpretation in the final draft report.