Farmers cannot always depend solely on rainfall to sustain their crops. In fact, many of the world's agricultural regions receive little or no precipitation. In these areas, water must be diverted from streams or lakes or drawn from wells. The result has been the development of irrigation systems. Mesopotamia, considered the cradle of civilization, had one of the earliest water diversion systems. Agriculture began there about ten thousand years ago. Water channels helped divert water from the Tigris and Euphrates Rivers during the dry months and also helped control the annual July floods. The civilization that developed in Egypt owed its existence to the water diversion schemes that delivered water to the fields from the Nile River and its delta. The ancient Egyptians invented the shaduf, a long pole with a bucket suspended at one end and a counterweight mounted at the other. The pole pivoted vertically to...