. 2019 Sep 19;10:4281. doi: 10.1038/s41467-019-12179-5

Table 1.

Metrics commonly used to evaluate GHG emissions in hydropower projects

Metric	Units	Description
GHG flux	kg CO₂eq km⁻² d⁻¹	The exchange of GHG, in CO₂ equivalents, at the reservoir air-water interface per unit of surface area over a certain time period. The direction of GHG flux can be from water to atmosphere (emission or efflux; positive value) or from atmosphere to water (uptake or influx; negative value).
Total GHG flux	Tg CO₂eq	GHG flux over a reference time period multiplied by the total reservoir area. The reference times considered here are a day and 1, 20, and 100 years (1 Tg = 10¹² g).
Power density	MW km⁻²	The ratio of electricity generation capacity to reservoir flooded area. This metric reflects the strong link between GHG emissions and flooded area and is often used as a simple proxy for carbon intensity.
Carbon intensity	kg CO₂eq MWh⁻¹	Also known as emission intensity or emission factor. CO₂-equivalent emissions produced per unit electricity generated. This metric is used to compare emissions performance across projects of different sizes, and also among electricity sources.