Quantify the impact of hydrological droughts on carbon dioxide emission from the Yangtze River networks

Rivers, as an important source of CO₂ emissions, release substantial amounts of CO₂ into the atmosphere through gas exchange at the water-air interface, profoundly influencing the global carbon cycle. In recent years, frequent drought events driven by global climate change have markedly impacted aquatic ecosystems. Hydrological drought, identified by prolonged low river discharge, can disrupt the transport and decomposition of organic matter, leading to pronounced effects on riverine CO₂ emissions. Nonetheless, the magnitude of this drought-induced alteration in CO₂ emission fluxes is still not fully understood. In this study, we investigated riverine CO₂ emissions in the Yangtze River networks, China, from 1979 to 2019 using the boundary layer method. We quantified the impact of hydrological droughts on riverine CO₂ emissions from the perspective of river classification. Results showed that hydrological droughts reduced CO₂ evasion by approximately 33% compared to non-drought periods. Specifically, CO₂ emission flux declined by 18.91%, 25.06%, 31.43%, and 43.22% under mild, moderate, severe, and extreme drought, respectively. River width contraction was identified as the dominant mechanism driving drought-induced reductions in CO₂ emissions. Our results showed that lower-order rivers exhibited larger CO₂ emission declines, while higher-order rivers showed smaller reductions. This study contributes to a more comprehensive understanding of the impact of hydrological droughts on riverine CO₂ emissions, while also providing useful insights for riverine carbon flux dynamics.