The deforestation of the Amazon is often referred to in terms of the loss of habitat and species. But it also may come back to cause unforeseen problems for us humans, based on a new study in PNAS. Most models of future hydropower productivity have assumed deforestation will lead to increased water runoff, which will in turn increase the amount of power that existing projects will generate. But the study suggests that the feedback between forests and rainfall will ultimately lead to a prolonged and more intense dry season, leaving hydroelectric plants generating less power.

Hydropower is a major contributor of renewable electricity in South America. 100 percent of Paraguay's electricity comes from hydro, and it's a major exporter of power. Brazil isn't far behind, meeting 80 percent of its electrical needs using hydropower, and with several major projects still in the works.

The study focuses on one of these projects which will tap into the potential of the Xingu river basin. A series of damns and hydroelectric facilities in the basin are slated to fill 40 percent of the increase in generating capacity that Brazil expects to need by the end of the decade.

Currently, estimates for its future production are that it will rise due to deforestation. That's because trees and other plants are very good at extracting moisture from the soil but not as good at retaining it in their leaves. The process by which ground water is released as water vapor by the leaves of plants is called evapotranspiration. Estimates are that the loss of forest will reduce evapotranspiration, raising the future production of electricity by the Xingu project by anywhere from four to 12 percent, depending on the degree of forest loss.

But the reduction of evapotranspiration isn't the only affect that cutting down forests will have. In recent years studies have found that tropical rainforests are a self-reinforcing system. The heavy rains enable forest growth and the forest itself feeds back in a way that encourages further rainfall.

Modeling this feedback, the authors show that a deforestation rate of 15 percent will drop the runoff in the Xingu by anywhere from six to 13 percent. If deforestation reaches 40 percent, then the declines will be dramatic, most probably over 30 percent. The declines won't be evenly distributed over the calendar, however, and will largely have the effect of prolonging the dry season, a time when the hydroelectric plants will largely be idle. (The project is only expected to be producing electricity at 40 percent of its potential capacity because of this.)

Overall, the authors calculate that the current rate of deforestation will drop the project from this 40 percent figure down to about 35 percent of electricity-generation capacity. That may not seem like much, but it represents about three percent of Brazil's total current capacity.

And things could potentially get even worse. The large construction project is attracting people to the area, which may increase the rate of deforestation. A number of climate models show the Amazon as a whole will get substantially drier as the century progresses, and the severe droughts of 2005 and 2010 seem to point in that direction as well. If these trends continue, the future could be even worse for the project.

Aside from having large practical implications, the paper's a nice example of how we can integrate more recent findings—the forest-cloud feedback—into existing models, and come out with what we hope is a slightly more accurate answer.

PNAS, 2013. DOI: 10.1073/pnas.1215331110 (About DOIs).