Basin-Scale Screening of Spillway-Related Total Dissolved Gas Generation Potential Under Intermittent Hydropower Operation in the Brazilian Amazon and Tocantins–Araguaia Basins

As variable renewable generation grows, intermittent dispatch of run-of-river hydropower plants may transfer required environmental-flow releases from turbines to spillways, creating conditions that favor total dissolved gas (TDG) supersaturation and may constrain hydropower flexibility. This study presents a reproducible basin-scale screening assessment for 24 selected hydropower plants in the Brazilian Amazon and Tocantins–Araguaia basins. The analysis combines a plant inventory, spillway typology, standardized environmental-flow scenarios, and two configuration-specific linear relationships between unit discharge and the increase in TDG saturation (ΔTDG), derived from digitized Pubugou and Gongzui observations. Two release configurations were examined: flow distributed among all available bays and flow concentrated in a single bay as a theoretical hydraulic bounding case. The Pubugou-based relationship was applied only to broadly comparable ski-jump configurations within 9.1 ≤ UD ≤ 72.3 m3 m−1 s−1, whereas the Gongzui-based relationship was provisionally assigned, as an inventory-level first-order analog, to controlled spillways discharging into stilling basins within 34.6 ≤ UD ≤ 234.6 m3 m−1 s−1. Hydraulically dissimilar cases were classified as NE-H, and cases outside the applicable empirical domain as NE-UD. Digitization sensitivity, regression uncertainty, and model-form selection were explicitly evaluated and documented. None of the distributed-flow scenarios produced a numerical estimate: cases assigned to the Pubugou- or Gongzui-based relationships were classified as NE-UD, whereas hydraulically dissimilar free-surface cases were classified as NE-H. Four single-bay scenarios produced configuration-specific ΔTDG increments: 22.5–33.4 percentage points for three Gongzui-based cases and 13.6 percentage points for one Pubugou-based case. Upstream TDG was not added, and no plant-specific final concentration or universal ranking was reported. Sinop and Colíder observations were retained as qualitative contextual evidence of limited transferability and the importance of site-specific hydraulics. The outputs support conditional monitoring prioritization under standardized assumptions, not compliance prediction, ecological-risk assessment, or gate-operation recommendations. Synchronized monitoring and plant-specific rating curves are required before TDG-related variables can be incorporated into operational planning.