Undervalued dry riverbeds: A key factor in equating intermittent river CO₂ emissions to perennial rivers

Intermittent rivers in semiarid and arid regions, constituting over half of the world's rivers, alternate the carbon cycle interactions among the biosphere, hydrosphere, and atmosphere. Inadequate quantification of flow duration and river water surface area, along with overlooked CO₂ emissions from dry riverbeds, result in notable inaccuracies in global carbon cycle assessments. High-resolution remote sensing images combined with intensive field measurements and hydrological modelling were used to estimate and extract the flow duration, river water surface area and dry riverbed area of Huangfuchuan, an intermittent river watershed that acts as a major tributary of the Yellow River in semiarid Northwest China. CO₂ emission rates and partial pressures in water and air across the watershed were in-situ measured. In 2018, the flow duration of Huangfuchuan increased from less than 5 days in the first-order tributary to 150 days in the sixth-order mainstream. River water surface area estimated by remote sensing extraction plus the hydrodynamic model simulation varied from 3.9 to 88.6 km² under 5 %-95 % discharge frequencies. CO₂ emissions from the water-air interface and dry riverbed in 2018 were estimated at 582.3 × 10³ and 355.2 × 10³ ton, respectively. The estimated total annual emission (937.5 × 10³ ton) aligns closely with the range of emissions (67.3 × 10³-1377.2 × 10³ ton) calculated for the water-air interface alone, derived using DEM river length and hydraulic geometry method. This similarity can be attributed to the overestimation of flow duration and flow velocity, as well as the over- or under-estimation of river water surface area and slope. The new method proposed in this study has large potential to be applied in estimating CO₂ emissions from data-scarce intermittent rivers located in mountainous regions and provides a standardized solution in the estimation of CO₂ emission. Results of this research reveal the spatiotemporal distribution of CO₂ emissions along an intermittent river system and highlight the substantial role of dry riverbed in carbon cycle.