Expert panel: A−R remains a useful grower metric while SWAT and hydrogeologic models inform regional targets
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Summary
At its Nov. 19 meeting the State Water Resources Control Board agricultural expert panel said A−R (applied minus removed) is a practical reporting metric for growers, while hydrogeologic tools such as the CV‑SWAT model can help coalitions and regional boards estimate leaching and set township‑level targets. Panelists emphasized regional flexibility and cautioned against mandating complex models where calibration or subsurface data are lacking.
The State Water Resources Control Board’s second statewide agricultural expert panel on Nov. 19 discussed how to measure and interpret nitrogen discharges to groundwater, balancing simple grower‑reportable metrics against more complex modeling approaches.
Chair Daniel Geisler, a cooperative extension specialist at UC Davis, said the panel agreed that applied minus removed (A−R) is “an appropriate metric to use” for regulatory reporting because it is accessible to growers and can be calculated from reported application and yield data. He cautioned, however, that A−R “does not capture” processes such as denitrification and volatilization that affect actual leaching.
Panelists said those processes are better estimated with hydrogeologic or watershed models. Kenneth Miller, one modeler who spoke during public comment, described the CV‑SWAT approach used in parts of the Central Valley, saying the tool “integrates additional datasets — soil, climate, irrigation — and connects to reported INMP data to provide estimates of leaching and the full nitrogen cycle.” He said coalitions are using SWAT at field scale and rolling results up to township targets.
But several panelists, including Sarah Lopez of Preservation Inc., warned that SWAT and its coupled subsurface models require calibration and basin‑specific hydrogeologic data that many regions — notably parts of the Central Coast — do not have. Lopez asked the panel not to mandate SWAT where calibration and funding are lacking. “Please don’t give us a regulatory mandate to use the SWAT model on the Central Coast because I don’t know if it’s going to work,” she said.
Panel members proposed a pragmatic division of labor: use A−R as the grower‑level reporting metric while allowing coalitions or third‑party modelers to run hydrogeologic models to estimate additional losses and to inform regional or township targets. “We don’t want growers to have to measure denitrification or volatilization themselves,” said Ruth Dahlquist Willard of UC ANR; models and coalition analyses can incorporate those processes for implementation and planning.
Panelists also discussed regional tailoring: SWAT may perform well in the Central Valley but not on the Central Coast or desert regions, so the panel recommended allowing regions to select appropriate models and to use SWAT as one example rather than a universal requirement.
Next procedural steps: panel members asked staff to reflect this layered approach in draft language and to clarify that A−R is suitable for regulatory reporting while models are tools for implementation and target‑setting. The panel will continue refining recommendations at its Dec. 5 meeting and expects to produce a consolidated draft for public review in January.