Diffusive emissions of methane and nitrous oxide from a cascade of tropical hydropower reservoirs in Kenya

摘要

The present study investigated diffusive emissions of methane (CH_4) and nitrousoxide (N_2O) to the atmosphere from three relatively small (3–120 km~2) reservoirs(Masinga, Kamburu and Gitaru) on the Tana River (Kenya). Sampling was conductedbiweekly in 2011, 2012 and 2013, at sampling sites upstream and downstream ofthese reservoirs while five sampling campaigns were carried out in 2011, 2012 and2013 for different sites within each of the reservoirs. The dissolved CH_4 (range:19–2101 nmol/L) and N_2O (range: 6.2–11.5 nmol/L) concentrations in the surfacewaters were generally very low in the three reservoirs, compared with other reservoirsglobally. The lower diffusive emissions of CH4 (20–216 μmol/m~2 day~(−1)) andN_2O (1.0–1.6 μmol/m~2 day~(−1)) from these reservoirs, compared with other tropicalreservoirs, are probably related to their age (30–40 years), and lower vegetation biomass(savannah) originally present and submerged during their commissioning. Thereservoirs with longer water residence times were characterized by higher diffusiveCH4 fluxes (216 ± 666 μmol/m~2 day~(−1)) and slightly lower N_2O fluxes (1.0 ± 1.5 μmol/m~2 day~(−1)). The relative contribution of turbine fluxes of CH_4 and N_2O, comparedto diffusive fluxes, was also highly variable among the three dams, being lower inMasinga Reservoir and higher in Gitaru Reservoir.