WHY IS WATER scarce and growing scarcer, approaching the status of a genuine shortage? In large part because we waste it. Anyone who has ever visited Los Angeles, for example, knows that the leaf rake is an endangered species there -- these days leaves are blown off lawns and driveways with a spray hose. That's called progress. But agriculture is the truly big waster: the General Accounting Office recently concluded that at least half of the water used for irrigation in this country is wasted.

And why does wasting water come so easy? In large part because of billion-dollar federal subsidies that provide water for irrigation at a fraction of its real cost. A utility in Utah, for example, recently paid $1,750 per acre-foot of water while not far away, in California's San Joaquin Valley, farmers were paying less than $10. In addition, there is increasing concentration of growth in areas where water supplies are lowest -- in the Sun Belt. Many of these booming cities, like Phoenix, for example, have been built on non-renewable supplies of ground water. Phoenix prides itself on having "made the desert bloom," but the price is a water table that has dropped 400 feet and more. With a measly 10 inches of annual rainfall, Arizona ranks among the top 10 states in per-capita water consumption.

In the East, where flooded basements are more common than falling water tables, a less familiar type of shortage may be developing. This is not an absolute shortage, but a shortage of usable water: water that is, in the vernacular of the trade, "fishable and swimmable." Some years ago Congress decided to try to reach that goal -- fishable and swimmable -- for the nation's waters by 1983. It won't be met, though not for want of trying. The construction of waste-water treatment facilities is now the nation's largest public works project, outdistancing even the pork-barrel dams and various Corps of Engineers boondoggles. But despite the nearly $30 billion in federal money that has been spent, the goal of clean water keeps receding. and the list of hazardous substances that should be removed from water to ensure safety grows.

The chairman of one congressional committee on water resources said the other day that "if we don't do something," the water shortage could make the energy crisis look like " a pink tea party." But there doesn't have to be a national water crisis. The country has about as much water available as it ever had -- about 600 billion gallons per day. It is simply being used faster than it can be replenished. And since part of the reason is the price, which has been held by federal policy way below water's real value, and part the wasteful practices for which there are many technological alternatives, the similarities to past mistakes with oil and gas should be too obvious to ignore. s