# The carbon footprint of a Malaysian tropical reservoir: measured versus modelled estimates highlight the underestimated key role of downstream processes

Reservoirs are important sources of greenhouse gases (GHGs) to the atmosphere, and their number is rapidly increasing, especially in tropical regions. Accurately predicting their current and future emissions is essential but hindered by fragmented data on the subject, which often fail to include all emission pathways (surface diffusion, ebullition, degassing, and downstream emissions) and the high spatial and temporal flux variability. Here we conducted a comprehensive sampling of Batang Ai reservoir (Malaysia), and compared field-based versus modelled estimates of its annual carbon footprint for each emission pathway. Carbon dioxide (inline-formulaCO2) and methane (inline-formulaCH4) surface diffusion were higher in upstream reaches. Reducing spatial and temporal sampling resolution resulted in up to a 64 % and 33 % change in the flux estimate, respectively. Most GHGs present in discharged water were degassed at the turbines, and the remainder were gradually emitted along the outflow river, leaving time for inline-formulaCH4 to be partly oxidized to inline-formulaCO2. Overall, the reservoir emitted 2475 inline-formula $M5inlinescrollmathmlunit\mathrm{normal g}\phantom{\rule{0ex}{0ex}}{\mathrm{normal CO}}_{normal 2}\phantom{\rule{0ex}{0ex}}\mathrm{normal eq}\phantom{\rule{0ex}{0ex}}{\mathrm{normal m}}^{-normal 2}\phantom{\rule{0ex}{0ex}}{\mathrm{normal yr}}^{-normal 1}$ 86pt16ptsvg-formulamathimgb72ab9da8fb2c27bb0ef6b10141be296 bg-17-515-2020-ie00001.svg86pt16ptbg-17-515-2020-ie00001.png , with 89 % occurring downstream of the dam, mostly in the form of inline-formulaCH4. These emissions, largely underestimated by predictions, are mitigated by inline-formulaCH4 oxidation upstream and downstream of the dam but could have been drastically reduced by slightly raising the water intake elevation depth. inline-formulaCO2 surface diffusion and inline-formulaCH4 ebullition were lower than predicted, whereas modelled inline-formulaCH4 surface diffusion was accurate. Investigating latter discrepancies, we conclude that exploring morphometry, soil type, and stratification patterns as predictors can improve modelling of reservoir GHG emissions at local and global scales.

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Soued, Cynthia / Prairie, Yves T.: The carbon footprint of a Malaysian tropical reservoir: measured versus modelled estimates highlight the underestimated key role of downstream processes. 2020. Copernicus Publications.

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