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  3. The majority of the population in the world might drink only two liters of water a day, but they consume about 3,000 liters a da

The majority of the population in the world might drink only two liters of water a day, but they consume about 3,000 liters a da

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问题     The majority of the population in the world might drink only two liters of water a day, but they consume about 3,000 liters a day if the water that goes into their food is taken into account. The rich gulp down far more, since they tend to eat more meat, which takes far more water to produce than grains. So as the world’s population grows and incomes rise, farmers will need a great deal more water to keep everyone fed; 2,000 more cubic kilometers a year by 2030, according to the International Water Management Institute(IWMI). Yet in many farming regions, water is scarce and likely to get scarcer as global warming worsens. The world is facing not so much a food crisis as a water crisis, argues Colin Chartres, IWMI’s director-general.
    The solution, Mr. Chartres and others contend, is more efficient use of water or, as the sloganeers put it, "more crop per drop". Some 1. 2 billion people live in places that are short of water. Farming accounts for roughly 70% of human water consumption. So when water starts to run out, farming tends to offer the best potential for thrift. But governments rarely charge farmers a market price for water. So they are usually more wasteful than other consumers—even though the value they create from the water is often less than households or industry would be willing to pay for it.
    The pressing need is to make water go further. Antoine Frerot, the head of the water division of Veolia Environment, promotes recycling of city wastewater to be used in industry or agriculture. This costs less and cuts pollution.
    Yet as Mr. Frerot himself concedes, there are many even cheaper ways to save water. As much as 70% of water used by farmers never gets to crops, perhaps lost through leaky irrigation channels or by draining into rivers or groundwater. Investment in drip irrigation, or simply repairing the worst leaks, could bring huge savings.
    " Farmers in poor countries can usually afford such things only if they are growing cash crops," says David Molden of IWMI. "Even basic kit such as small rainwater tanks can be lacking. " Ethiopia, for example, has only 38 cubic meters of storage capacity per inhabitant, compared to almost 5,000 in Australia. Yet modest water storage can hugely improve yields in rain-fed agriculture, by smoothing over short dry spells. Likewise, pumping water into natural aquifers for seasonal storage tends to be much cheaper than building a big dam, and prevents the great waste of water through evaporation.
    Agronomists are beginning to devise tools to help monitor the efficiency of water use. Some have designed algorithms that use satellite data on surface temperatures to calculate the rate at which plants are absorbing and transpiring water. That allows governments and development agencies to concentrate their efforts on the most prodigal areas.
    Raising yields does not always involve greater water consumption, especially when farms are inefficient. It would take little extra water to double cereal output in many parts of Africa, Mr. Molden argues. IWMI reckons that some three-quarters of the extra food the world needs could be provided simply by bringing yields in poor countries closer to those of rich ones. That is more realistic than the absolute alternative: giving up meat and other thirsty products altogether.
   
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答案C
解析 题目对应信息在第五段最后一句：Likewise，pumping water into natural aquifers for seasonal storage tends to be much cheaper than building a big dam，and prevents the great waste of water through evaporation．大意是“同样，将水抽到自然蓄水层比建水坝便宜得多，并能防止蒸发造成的浪费”。因此建水坝效果差是因为费用和效率问题。
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