Globally,
irrigation increases agricultural productivity by an amount roughly equivalent
to the entire agricultural output of the United States, 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 to 2002, estimating the amount of carbon uptake enabled by
relieving water stress on croplands. The results are published in Global Biogeochemical Cycles.
“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 maximizes 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.
The study takes
an important step toward quantifying how management decisions can impact global
carbon balance and assessing the economic worth of water and carbon in
irrigated landscapes.
“Now that
we have spatially-explicit maps of how much irrigation is increasing carbon
accumulation, we have good information about the value of the water going into
those areas. We might be able to come up with a value of carbon in those areas
as well,” he says. “Of course the flip side of this is that, in many
places around the world, if we keep irrigating we are either going to run out
of water or degrade soils because of salinity issues.”
The current
study does not factor in any impacts in areas from which irrigation water is
drawn. However, Ozdogan says, a better understanding of the links between
irrigation, productivity, and carbon will help researchers look at downstream
effects of factors that influence each of those elements—for example, how water
shortages in agricultural regions may affect regional carbon cycles and
climate.
