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

Abstract

The present study investigated diffusive emissions of methane (CH4) and nitrous oxide (N2O) to the atmosphere from three relatively small (3–120 km2) reservoirs (Masinga, Kamburu and Gitaru) on the Tana River (Kenya). Sampling was conducted biweekly in 2011, 2012 and 2013, at sampling sites upstream and downstream of these reservoirs while five sampling campaigns were carried out in 2011, 2012 and 2013 for different sites within each of the reservoirs. The dissolved CH4 (range: 19–2101 nmol/L) and N2O (range: 6.2–11.5 nmol/L) concentrations in the surface waters were generally very low in the three reservoirs, compared with other reservoirs globally. The lower diffusive emissions of CH4 (20–216 µmol/m2 day−1) and N2O (1.0–1.6 µmol/m2 day−1) from these reservoirs, compared with other tropical reservoirs, are probably related to their age (30–40 years), and lower vegetation biomass (savannah) originally present and submerged during their commissioning. The reservoirs with longer water residence times were characterized by higher diffusive CH4 fluxes (216 ± 666 µmol/m2 day−1) and slightly lower N2O fluxes (1.0 ± 1.5 µmol/m2 day−1). The relative contribution of turbine fluxes of CH4 and N2O, compared to diffusive fluxes, was also highly variable among the three dams, being lower in Masinga Reservoir and higher in Gitaru Reservoir.