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Harvest and save water to increase yields

October 16, 2009  By Potsdam Institute for Climate Impact Research (PIK)

October 15, 2009 – On-farm
water management could increase global crop production by about one fifth, a
modeling study by German and Swedish researchers indicates.

October 15, 2009 – On-farm
water management could increase global crop production by about one fifth, a
modeling study by German and Swedish researchers indicates.

However, even intensive
water management on present cropland will not be sufficient to accommodate the
food demands of a growing population in a warming world, the scientists report
in the current edition of “Environmental Research Letters.”


“Use of water in
agriculture is a key problem for the 21st century: without improvements neither
the consequences of climate change will be manageable nor the demand of two or
three billion additional people for food be met,” says Wolfgang Lucht of the
Potsdam Institute for Climate Impact Research (PIK). “In this study we
therefore investigated whether there are realistic opportunities to close the
emerging gap in water supply for agriculture at least partially for many world
regions. The results are quite encouraging.”

Today, about 15 million
square kilometres, roughly ten per cent of the total land surface, is covered
by cropland. An earlier study by the researchers suggested that without
substantial improvements in water productivity or other measures to increase
yields on present cropland, an expansion by about ten million square kilometres
would be required if the world population rose to 10 billion in 2050 as
suggested by the IPCC’s SRES A2r scenario. The yearly consumption of freshwater
for irrigated and rainfed agriculture would have to be increased by an
additional 4500 cubic kilometres from the current 8800 cubic kilometres.

“However, in many regions
of the world that already face limits of water availability, that is not an
option,” says Dieter Gerten, a hydrologist at PIK. “Instead, we need to think
of better ways to use the water that is there.”

The research team headed
by Gerten investigated how additional land and water requirements could be
minimized through water management on existing cropland. The study, based on
simulations with a vegetation-water model, quantifies the potentials of two
water management strategies for increasing crop production: harvesting
rainwater for use during dry spells and reducing soil evaporation.

In practice, a vapour shift
from unproductive soil evaporation to productive plant transpiration that
permits biomass growth can be attained through mulching or applying different
tillage systems. Field studies show that soil evaporation can be halved this
way. The researchers estimate the potential to increase global crop yield to
amount to two to 25 per cent, depending on management intensity. The highest
potentials of more than 20 per cent for a moderate management regime lie mainly
in semiarid regions, such as the Midwestern United States, the Sahel, Southern
Africa, and Central Asia.

Rainwater can be harvested
by concentrating and storing runoff in ponds, or with the help of dikes or
subsurface dams. The water can be redirected to crops in periods of water
stress so the risk of crop failure is reduced. In the current study, water
harvesting was simulated to increase global crop yield by four to 31 per cent,
again depending on management intensity. With moderate management intensity,
parts of South America and parts of Africa show large potentials of more than
20 per cent for increasing crop yield.

The combination of both
management strategies would result in a production increase of seven to 53 per
cent. Pronounced increases can be achieved mainly in regions where present yields
reach less than one tenth of what could theoretically be reached if water
supply was unlimited, as in large parts of Africa. Globally, a moderate and
feasible management scenario suggests that crop production can be increased by
19 per cent, which is comparable with the effect of current irrigation that
amounts to 17 per cent.

“However, the detrimental
effects of climate change could reduce global crop production by almost 10 per
cent by 2050,” says Stefanie Rost of PIK. Even if the beneficial effects of the
rising atmospheric concentration of carbon dioxide on plant growth and the
moderate water management scenario were realized, the water available on
current cropland would not meet the requirements of a world population of nine
or ten billion.

“This evidence poses
crucial questions about tradeoffs between future land and water use for
irrigated and rain-fed agriculture, natural ecosystems and bioenergy,” the
authors state. They suggest exploring options of more efficient irrigation and
expansion of irrigated agriculture, of plant breeding and genetic engineering,
and of more effective trade with agricultural products from water-rich to
water-poor regions.

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