Emissions of greenhouse gases from the reservoirs of hydroelectric dams: Implications of a power law

Abstract

Hydroelectric dams emit greenhouse gases, especially methane (CH 4), which is produced by decomposition of organic matter under anoxic conditions at the bottom of the reservoirs. A part of this gas is released by bubbling and diffusion through the surface of the reservoir, and part from the water that passes through the turbines and spillways. The portion of the emission that occurs through the reservoir surface has been calculated in estimates by the Brazilian government agency responsible for energy planning. The official calculation used a power law that resulted in an estimate of CH 4 76% lower than a corrected "basic" mean that represents the arithmetic mean of the measurements that served as the basis of the calculation. Expressed in comparison to the official value, the approximation of the arithmetic mean is 320% higher. We show that the adjustment that was applied in the official estimates was based on several mathematical errors, and that the real value should be higher, rather than lower, than the arithmetic mean. We compared various possible methods for generating a "corrected" estimate, all with results far above the official values. A best method was identified that indicates a reservoir-surface emission 345% higher than the official value. For Brazil's 33×10 3 km 2 of reservoirs, the total impact of the underestimate of surface emissions of CH 4 is almost as large as the emission produced by burning fossil fuels in greater São Paulo, while the total emission of the reservoir surfaces surpasses the emission of this city. Emissions from the water that passes through the turbines and spillways represent an additional impact on global warming.