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Expert panel affirms A minus R as useful field metric but urges regional modeling, audit and phased reporting
Summary
At a Nov. 19 State Water Resources Control Board panel meeting, experts said A minus R is a practical field-scale metric for estimating nitrate discharge but recommended using regional hydrogeological models (e.g., SWAT) for broader assessment, stepped implementation across regions, strengthened audit procedures and clearer reporting guidance for growers.
Daniel Geiser, a UC Cooperative Extension specialist and chairing the panel, opened a Nov. 19 working session of the State Water Resources Control Board’s expert panel, which focused on draft responses to technical questions about nitrogen mass-balance metrics and modeling.
Panelists broadly agreed that the metric A minus R (applied nitrogen minus nitrogen removed) is a valid, accessible field-scale measure for estimating potential nitrate discharge and for communicating performance among growers. “A menos R es una métrica válida a utilizar para estimar o calcular los descargas potenciales de nitrato,” the panel summary stated, reflecting the group’s view that the metric can be practical for comparisons and for informing growers. At the same time, panelists cautioned that A minus R does not capture all subsurface processes (for example, denitrification) and should be complemented by hydrogeological modeling where appropriate.
Experts discussed the regional limits of the…
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