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Industry, too, needs water. It takes about 22% of the world's withdrawals. Domestic activities take the other 8%. Together, the demands of these two categories quadrupled in the second half of the 20th century, growing twice as fast as those of farming.
工業同樣需要用水。工業用水約占世界總用水量的22%。家庭用水占另外的8%。這兩種用水加起來的總量在20世紀后半葉翻了兩番，其增長速度是農業用水增長速度的兩倍。
Meeting that demand is a difficult task. One reason is that the supply of water is finite. The world will have no more of it in 2025 or 2050 than it has today, or when it lapped at the sides of Noah's Ark. This is because the law of conservation of mass says, broadly, that however you use it, you cannot destroy the stuff. Neither can you readily make it. If some of it seems to come from the skies, that is because it has evaporated from the Earth's surface, condensed and returned.
滿足上述的用水需求是個艱巨的任務。原因之一是水的總董是有限的。無論是到2025年還是到2050年，地球上的水不會比今天的水多，也不會比當年輕拍諾亞方舟船舷的水多。這是因為根據質量守恒定律，大體上而言，無論怎么用，你都消滅不了水這一物質。當然你也不容易制造出這一物質。如果說一部分水好像來自天空，那是因為它從地球表面蒸發、濃縮、又重新降落到地球表面的緣故。
Most of this surface is sea, and the water below it—over 97% of the total on Earth—is salty. In principle, the salt can be removed to increase the supply of fresh water, but at present desalination is expensive and uses lots of energy.
地球的表面大部分是海洋，海洋里的水——占總量的97%以上——是咸的。理論上，可以除掉海水中的鹽分，以增加淡水供應量，但目前海水淡化既昂貴又需要消耗大量能源。
