People can imagine life without water. Without it, the earth we live on would be a dead one. There would be no trees, no crops, no animals and no people. So water is considered to be next to in importance.

    Although nearly three quarters of the earth's surface is covered with water, it is sea water and . The demand for water is increasing, while water are scarce.

    Agricultural production needs enormous amounts of water, so does industry. Besides, the rapid growth of population makes it more and more difficult for people tobecause of the water shortage. So water shortage has become one of the most serious problems.