Researchers quantify impacts of irrigation on global carbon uptake

Globally, irrigation increases agricultural productivity by an amount roughly equivalent to the entire agricultural output of the US, according to a new University of Wisconsin-Madison study.

That adds up to a sizeable impact on carbon uptake from the atmosphere. It also means that water shortages - already forecasted to be a big problem as the world warms - could contribute to yet more warming through a positive feedback loop.

The new research quantified irrigation's contribution to global agricultural productivity for the years 1998-2002, estimating the amount of carbon uptake enabled by relieving water stress on croplands. The results published August 25 in the journal Global Biogeochemical Cycles (which can be found at http://www.agu.org/journals/gb/gb1103/2009GB003720/), a publication of the American Geophysical Union.

"If you add up all the annual productivity that comes solely due to irrigation, it adds up to about 0.4 petagrams of carbon, nearly equivalent to the total agricultural productivity of the United States," says study author Mutlu Ozdogan, a UW-Madison professor of forest and wildlife ecology and member of the Nelson Institute for Environmental Studies.

The study also shows quantitatively that irrigation increases productivity in a nonlinear fashion - in other words, adding even a small amount of water to a dry area can have a bigger impact than a larger amount of water in a wetter region. "More irrigation doesn't necessarily mean more productivity," Ozdogan says. "There are diminishing returns."

This was already known on the field scale, he says, but is true globally as well. Interestingly, he found that, on average, worldwide irrigation is currently conducted close to the optimal level that maximises gains. While this may be good news for current farmers, it implies limited potential for irrigation to boost future productivity even as food demands increase.