The Carbon and Water Footprint of Hydropower

Hydropower is the largest renewable electricity source, but despite its reputation of being clean, it can have negative impacts on water resources and the climate. Since, in contrast to most energy technologies, the impacts of... [ view full abstract ]

Hydropower is the largest renewable electricity source, but despite its reputation of being clean, it can have negative impacts on water resources and the climate. Since, in contrast to most energy technologies, the impacts of hydropower greatly depend on the location, water and carbon footprints of hydropower plants need to be improved to more accurately assess their impacts and capture the spatial differences.

Based on a global dataset of almost 1500 hydropower plants, representing 43% of annual hydroelectricity generation in 2009, we estimated the two footprints for that year. Where a reservoir fulfils other purposes besides hydropower, hydropower’s ranking among the multiple purposes was used to allocate only parts of the impacts to it. Global averages were weighted by the plants’ annual hydropower production.

Statistical modelling allowed estimating carbon footprints. The model was trained with a dataset of about 100 hydropower plants. The reservoir area-to-electricity ratio, the area, the age, the erosion rate and the maximum temperature served as predictors in generalized linear modelling. The final model was selected by multi-model inference, and then applied to the larger global dataset.

For estimating the water scarcity footprint according to ISO 14046, we derived the water consumption of hydropower from the reservoir’s monthly net evapotranspiration and storage changes. Water scarcity footprints were then calculated by multiplying with monthly water scarcity indices. If water is stored in wet seasons and subsequently released in dry seasons, benefits can be attributed to hydropower. If the opposite applies, impacts are higher than expected based on an annual water balance.

Paradoxical as it is, earlier studies usually overestimated hydropower’s impacts on water resources, while underestimating its contribution to global warming. Some plants are even beneficial and reduce water scarcity, which is indicated by negative water scarcity footprints. Our results further suggest that carbon footprints of hydropower can be substantial, with higher contributions from carbon dioxide than methane.

Globally, hydropower’s carbon footprint amounts to 76 g CO_2e / MJ (112 g CO_2e / MJ without allocation), and its water scarcity footprint is 6.9 m³ H₂O_e / GJ (9.2 m³ H₂O_e / GJ without allocation). However, both footprints show a high variation among individual plants. Many plants have trade-offs, and more of them accelerate climate change (494) than increase water scarcity (110). Only 200 of the analyzed plants do not cause adverse impacts in neither category, while 669 plants cause adverse impacts in both.

In conclusion, our results question the environmental sustainability of hydropower, and demonstrate that each plant needs to be scrutinized individually. Furthermore, based on our detailed assessment, we provide average carbon and water footprint results for hydropower in national electricity mixes, which help to better assess sustainable production, especially of electricity intensive products and consumption patterns.