According to a recent United Nations report, about 2.3 billion people-or 40 percent of the world’s population-surfer from some form of water shortage, depending upon the year or the season. These shortages will likely grow worse. Half of the water now used for agriculture-mainly for the irrigation of crops-is lost through evaporation; much of the world's surface water is being contaminated by pollution; and the underground water upon which one-fourth of the world’s population depends is increasingly receding, causing the water tables to sink, the ground elevation to drop, and, in areas adjacent to the ocean, seawater to mix with and foul] the ground water.    Other factors are adding to the problem. For example, the disruptions caused by global warming are adversely affecting the water cycle by which evaporated moisture, rainfall, surface water, and ground water are linked in a complex system. While these changes sometimes result in increased rainfall, the increase can actually make matters worse. Environmental damage-such as stripping away of forests and octet natural vegetation-has left many areas unable to handle more water. Increased rainfall simply causes flooding and erosion in which water available for human use escapes through runoff.    Perhaps the first step to sensible water policies and reasonable water use is for people to understand that the amount of water actually available to human beings on earth-the so-called Water Planet-is far smaller than commonly believed. Seawater makes up 97.5 percent of the water found on the earth's surface, and most fresh water exists in the form of polar ice, or, to a lesser degree, as deep underground water, neither of which are readily available for human use. Indeed, if the earth were shrunk to one meter in diameter, the equivalent amount of fresh available water would be a single spoonful. Without understanding that water is already in short supply, people and governments may never be able to reshape water usage to preserve our most precious of resources.