Spatially Resolved Measurements of CO₂ and CH₄ Concentration and Gas-Exchange Velocity Highly Influence Carbon-Emission Estimates of Reservoirs

Abstract

The magnitude of diffusive carbon dioxide (CO₂) and methane (CH₄) emission from man-made reservoirs is uncertain because the spatial variability generally is not well-represented. Here, we examine the spatial variability and its drivers for partial pressure, gas-exchange velocity (k), and diffusive flux of CO₂ and CH₄ in three tropical reservoirs using spatially resolved measurements of both gas concentrations and k. We observed high spatial variability in CO₂ and CH₄ concentrations and flux within all three reservoirs, with river inflow areas generally displaying elevated CH₄ concentrations. Conversely, areas close to the dam are generally characterized by low concentrations and are therefore not likely to be representative for the whole system. A large share (44-83%) of the within-reservoir variability of gas concentration was explained by dissolved oxygen, pH, chlorophyll, water depth, and within-reservoir location. High spatial variability in k was observed, and k_CH₄ was persistently higher (on average, 2.5 times more) than k_CO₂. Not accounting for the within-reservoir variability in concentrations and k may lead to up to 80% underestimation of whole-system diffusive emission of CO₂ and CH₄. Our findings provide valuable information on how to develop field-sampling strategies to reliably capture the spatial heterogeneity of diffusive carbon fluxes from reservoirs.