Spatially Resolved Measurements of CO and CH Concentration and Gas-Exchange Velocity Highly Influence Carbon-Emission Estimates of Reservoirs.

The magnitude of diffusive carbon dioxide (CO) and methane (CH) 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 CO and CH in three tropical reservoirs using spatially resolved measurements of both gas concentrations and k. We observed high spatial variability in CO and CH concentrations and flux within all three reservoirs, with river inflow areas generally displaying elevated CH 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 was persistently higher (on average, 2.5 times more) than k. Not accounting for the within-reservoir variability in concentrations and k may lead to up to 80% underestimation of whole-system diffusive emission of CO and CH. Our findings provide valuable information on how to develop field-sampling strategies to reliably capture the spatial heterogeneity of diffusive carbon fluxes from reservoirs.