Basin‐Scale CO2 Emissions From the East River in South China: Importance of Small Rivers, Human Impacts and Monsoons

Abstract

AbstractRiverine carbon dioxide (CO2) emissions are an essential component of the riverine carbon cycle, but an accurate assessment of riverine CO2 emission fluxes is still hindered by the spatial and temporal variations among river basins caused by differences in climate, watershed characteristics, and human activity. Here, we evaluate the riverine CO2 flux from the subtropical East River Basin (ERB) in south China, a region strongly affected by monsoon climate and anthropogenic land use changes. Our results suggest small rivers are major contributors to riverine CO2 emissions, even with relatively low CO2 concentrations and small water surface areas (SAs). They contribute disproportionately to 74.4% of the total fluxes due to high gas transfer velocity (k) across the water‐air interface. Land use changes have substantially enhanced CO2 emissions from river networks. Normalized areal riverine CO2 fluxes in the urban‐ and cropland‐dominated Middle and Lower ERB (27.6 and 39.4 g C m−2 yr−1) were two and three times higher than the 9.1 g C m−2 yr−1 in the forest‐dominated Upper ERB. Due to the larger water SA and higher k caused by monsoon‐induced precipitation, the East River acts as a stronger carbon source during the wet season, emitting 0.67 Tg C yr−1 to the atmosphere, which is about twice that during the dry season (0.33 Tg C yr−1). Our study illustrated how monsoon climate and land use in the ERB have regulated its riverine CO2 emissions. Our findings also provided valuable insights into the role of small rivers in the basin‐wide carbon cycle.