Researchers say Central Valley ‘‘30% decline’’ in applied-minus-removed nitrogen is not statistically supported
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Summary
Independent analysts told the State Water Resources Control Board expert panel that a reported 30% drop in Central Valley applied-minus-removed (A–R) nitrogen between 2019 and 2023 does not hold up to scrutiny because of incomplete reporting, year-to-year variability and outliers in the dataset.
A claim that A–R nitrogen loads across the Central Valley fell roughly 30% between 2019 and 2023 drew sharply different interpretations during a State Water Resources Control Board listening session.
Researchers and advocates told the second statewide agricultural expert panel that the 30% figure cited by regional staff is misleading. "An estimate of the 30% decline ... is misleading because the years for which the trend was calculated was cherry picked," hydrologist Ira Stewart Frey said, adding that extending the window back to 2018 reduces the magnitude of the change to about 10%.
Why it matters: regulators and the public are watching A–R numbers to decide whether numeric limits, voluntary targets or different enforcement paths are needed to protect groundwater and surface water from excess nitrogen. Panel members were told the current Central Valley dataset contains significant gaps and that many local reporting units (townships) lack the multi-year records needed to detect a reliable trend.
What speakers said and showed - Dr. Jake D’Alessandro (California Rural Legal Assistance / Santa Clara University) and Ira Stewart Frey analyzed the Central Valley Water Board's township-level data. They found that most township blocks show no statistically significant trend in A–R and that many townships report only one or two years of data. D’Alessandro said median and mean metrics change substantially if earlier years are included: "If you extend that time period to 02/2018, we actually see a much more modest drop." - D’Alessandro and Frey reported that fewer than half of township units reported A–R values for every year in the study window. They said 351 of the Central Valley townships either had insufficient data for trend analysis or showed no significant trend; only a small fraction of townships showed statistically significant increases or decreases. - Central Valley Water Board staff defended the use of 2019 as a baseline in part because reporting requirements and program coverage increased in 2019; panel members were encouraged to ask staff for underlying data and methods for independent verification.
Limits of the current record: presenters noted several factors that can bias trend estimates, including missing acreage reporting, apparent one-year spikes that may reflect data-entry errors (for example unusually high reported removal numbers for some orchard crops), and economically-driven year-to-year crop rotations that change the crop mix in particular townships. "There are large data gaps and unchecked unbelievable data and large outliers that preclude any statistical relevance of the findings," Frey told the panel.
Panel next steps: multiple commenters urged the panel and staff to publish field-level acreage linked to reported A–R values and to make transparent the methods used to compute regional trend statistics. Community advocates said access to parcel- or field-level data is necessary to identify local hotspots that endanger drinking water wells.
Ending: Panel members flagged the finding as a priority for the panel’s working groups and requested clearer, peer-reviewable methods and the underlying datasets so trend claims can be independently replicated.
