The start of work on the Belo Monte dam project, via minplanpac/Flickr
That the output of hydropower projects is likely to decline in most regions as our climate changes is a given. A new piece in Proceedings of the National Academy of Sciences paints a fuller picture of how localized changes in weather in Brazil brought about by deforestation could dramatically slash the output of the highly controversial 11.2 gigawatt Belo Monte Dam.
Taking into account both the direct and indirect effects of deforestation in the Xingu River basin of the Amazon, the researchers found that under business-as-usual scenarios for deforestation through 2050, the power generation of Belo Monte could just be 25 percent of its total installed capacity due to changes in water availability.
Even under current conditions output of the dam is likely to be under 50 percent of installed capacity in all but three to five months of the year, the study found.
It's generally thought (somewhat sadly, considering its greater impact) that deforestation in a watershed will increase the output of a hydroelectric project, due to declines in evapotranspiration, resulting from lower forest cover, outweighing any declines in precipitation associated with with forest loss.
This research confirmed that deforestation in the Xingu basin would increase water discharge by four to eight percent (assuming 20 percent deforestation) or 10 to 12 percent (assuming 40 percent deforestation). However, taking into account the decreased precipitation resulting from that deforestation, the balance tipped strongly in the other direction, with river discharge decreasing anywhere from six to 36 percent.
Study co-author Marcos Costa notes,
These results are extremely important for long-term energy planning. We are investing billions of dollars in hydropower plants around the world. The more rainforests are left standing, the more water we'll have in the rivers, and the more electricity we'll be able to get from these projects.
Though this report looks at one specific region in the Amazon, it does also note that similar relationships between forest cover and rainfall exists across the Amazon, as well as in rainforests in Central Africa and Southeast Asia. In all three locations there is a considerable push to expand hydropower.
In general, the report reminds us that, "The potential of regional deforestation to inhibit rainfall sufficiently to constrain energy generation is greatest where rainfall seasonality is already pronounced, and where deforestation is expected to be the greatest."
Over 90 percent of the electricity consumed in Brazil comes from hydropower, with roughly 1000 GW of untapped hydropower potential outside of areas set aside for conservation or otherwise off-limits. But even if they're outside of protected areas, tapping into those hydropower reserves isn't necessarily a good idea, considering the significant environmental and climate change effects of building large dams, particularly in tropical forests. Additionally, this new study suggests that estimates of hydropower potential might be optimistic.