China's scarce water supply is being wasted as crops grown in water-stressed provinces are exported to wet, rainfall-rich areas, a study reports.
Farming accounts for about 65% of water use in China and the limited resource is coming under pressure from rapid urbanisation and industrialisation.
Officials have called the nation's water shortage a "grave situation" and called for strict water controls.
"China faces most of the major challenges to sustainable agriculture," wrote an international team of researchers.
"Fast socioeconomic development, rapid urbanisation and climate change, along with very limited water resources and arable land per capita," they added.
"Because arable land is available mainly in the water-scarce north, irrigation has become widespread, covering 45% of the country's agricultural land and accounting for 65% of national water withdrawal.
The study focused on four major food crops - soya, wheat, rice and corn (maize) - and three livestock groups: ruminant, pork and poultry.
Together, these products accounted for 93% of China's domestic food supply in 2005, according to figures from the United Nations.
The team - involving scientists from the US, Japan and China - assessed the volume of water used by different provinces to produce these crops and livestock, including the volume from rainwater and irrigation systems.
They concluded: "China's domestic food trade is efficient in terms of rainwater but inefficient regarding irrigation, meaning that dry, irrigation-intensive provinces tend to export to wetter, less irrigation-intensive ones.
"We (also) identify specific provinces (for example, Inner Mongolia) and products (for example, corn) that show high potential for irrigation productivity improvements.
The team added that the paper's findings had important policy implications.
"They constitute an essential input for designing policies and provide a framework for analysing how these policies might change China's… irrigation use in the near future."
The issue of water scarcity is one that the nation's officials know could undermine efforts to achieve sustainable development.
In 2012, China's vice minister of water resources, Hu Siyi, warned: "Because of the grave situation, we must put in place the strictest water resources management system."
He said that about two-thirds of Chinese cities were "water-needy", nearly 300 million rural residents lacked access to safe drinking water, and 40% of rivers were seriously polluted.
Although China has one of the world's largest annual internal renewable water resources (water in rivers and groundwater from rainfall), its population of 1.3bn people means that, per capita, the national average of renewable water supplies is just one third of the global average.
Within the country, there is a vast difference. People in northern provinces only have a tiny fraction of water resources available to them compared with residents of southern parts of China.
While the north only has about one-fifth of China's water supplies, it accounts for two thirds of the nation's cropland.
A report by the UN Food and Agriculture Organization (FAO) said the intensive use of groundwater resources had resulted in the lowering of water tables by up to 300m and the rapid depletion of groundwater reservoirs.
It added that groundwater extraction in northern provinces was unsustainable as water was being consumed faster than it could be replenished.
However, in contrast - it added - less than 30% of the known groundwater resources in southern China were being used as a result of having a more plentiful supply of surface water sources.