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Expert panel: A−R remains a useful grower metric while SWAT and hydrogeologic models inform regional targets
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…
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