Central Valley presenters show modeled progress in A−R but say groundwater response remains uncertain

Eric Warren, supervising water resources control engineer and program manager for the Central Valley Regional Water Quality Control Board, told the expert panel that coalition-submitted INMP data show a statistically significant decline in median A−R values from about 59 to 41 pounds per acre between 2019 and 2023.

The nut graf: Warren said the Central Valley has reduced its median A−R by roughly 30 percent, a change he described as measurable but not yet attributable to specific management practices; he and coalition modelers said more data over wet and dry cycles and additional ground‑truthing are needed before conclusions about groundwater improvement can be drawn.

Warren said the Central Valley regulates about 5.4 million enrolled acres through multiple general waste discharge requirements and 14 approved coalitions. He presented boxplots showing A−R distributions and a‑to‑R ratios and noted the region saw a 30 percent reduction in median A−R from about 59 to 41 pounds per acre since 2019. “We are making measurable progress, even in the absence of enforceable numeric limits,” Warren said, but he cautioned that cause‑and‑effect is difficult to establish because many changes occur simultaneously.

David Corey, a farmer and attorney who has worked with Central Valley coalitions, described how CV‑SWAT outputs feed into CBN‑PSAT to estimate the portion of applied nitrogen that moves past the root zone and the township‑level values used to estimate groundwater protections. Corey described the Nitrate Control Program and the management-zone approach developed through CV‑SALTS, which aims to provide replacement water and coordinated mitigation where wells are already contaminated while longer‑term load reductions proceed.

Panelists asked about linking the reductions in A−R to observed groundwater trends. Warren replied that groundwater response is complex, variable by depth and travel time, and that the Central Valley Groundwater Monitoring Collaborative is compiling monitoring data; he said it is too early to identify a region‑wide groundwater response attributable to recent A−R declines. Ending: Central Valley presenters asked the expert panel to use long‑run, field‑level and township‑level analyses to assess where limits or targeted interventions would most effectively reduce nitrate loading in basins with the highest human‑health risk.