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Harvest and save water to increase crop yields, say researchers
08 October 2009
Potsdam Institute for Climate Impact Research (PIK)
On-farm water management could increase global crop production by about one fifth, a modelling 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,” adds Lucht.
Today, about 15 million square kilometres, roughly ten percent 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 ten 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 currently 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, 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 2 to 25 percent, depending on management intensity. The highest potentials of more than 20 percent 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 that the risk of crop failure is reduced. In the current study, water harvesting was simulated to increase global crop yield by 4 to 31 percent, again depending on management intensity. With moderate management intensity, parts of South America and parts of Africa show large potentials of more than 20 percent for increasing crop yield.
The combination of both management strategies would result in a production increase of 7 to 53 percent. 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 percent, which is comparable with the effect of current irrigation that amounts to 17 percent.
“However, the detrimental effects of climate change could reduce global crop production by almost ten percent 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 realised, 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 rainfed 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.