The acid neutralizing capacity gap between two flow levels had an uncertain trend at Hubbard Brook watershed 6 in 2012–2024

We tested whether the acid neutralizing capacity (ANC) gap between two stream flow levels became smaller after 2011 at Hubbard Brook reference watershed 6, New Hampshire, USA. We used 1,523 selected weekly samples in 1994–2024 from HBWatER release knb-lter-hbr.208.14. The two flows were 0.81 and 3.89 liters per second, the sample quartiles. The model had separate year effects, month effects by period, time terms for the log-flow slope, and terms for two ANC instruments. The estimated change in ANC at the upper flow minus ANC at the lower flow was −1.84 microequivalents per liter from mid-2012 to mid-2024. Its nominal 95% three-year block interval was −6.33 to +3.91. A positive change would mean a smaller deficit at the upper flow. All seven further model and data checks gave intervals that included zero. Recorded annual ANC means increased, but the direction of the later change in the flow gap was uncertain. The interval does not prove equal recovery rates. This test uses routine-sample flow quartiles at one site. It does not estimate extreme storm exposure or biological recovery. Changes in collection and instruments remain limits. The package has unchanged source tables, provenance, a full sample ledger, all results, figures, offline code and independent numerical checks. A clean offline repeat matched all result and figure files.

acid neutralizing capacity · Hubbard Brook · stream chemistry · acid deposition · flow · recovery · reproducibility · uncertain trend