Although hikers sometimes try, man cannot live by granola bars alone. Hikers out here like to live by nature's sweetest drink, water dipped from high mountain streams. But even that may be becoming problematic.

It is not surprising that fish in Adirondack lakes are being killed by rain which, falling through the tangy air of the Northeast, acquires the acidity of lemon juice. But now some high Colorado mountain lakes and streams receive rain and snow with an acidity almost as strong as raw vinegar.

So even clean-air policies are, in a sense, water policies. And water policies will become national preoccupations in this decade.

One-quarter of America's water use depends on ancient underground deposits--aquifers-- built up over millennia but depletable in decades. The nation's estimated 59,000 trillion gallons of ground water are 45 times the volume of Lake Michigan, and many times the amount of water that has flowed from the Mississippi into the Gulf of Mexico in the last 200 years. But between 1950 and 1980, annual consumption of ground water more than doubled to about 25 trillion gallons. Every day the nation takes 21 billion gallons more from the ground than seeps back into deposits.

The thickness of major waterbeds varies from 200 to 1,000 feet, and in Texas and Kansas the water table has been falling between two and five feet a year. California farmers are pumping 2 million acre-feet (652 billion gallons) more than is replaced each year. In parts of California's San Joaquin valley, the land has sunk 29 feet as ground water has been pumped, and the compression of the land makes it impossible to fully recharge the aquifer.

As fresh water drains from aquifers, salt water from oceans or underground salt deposits can seep in. Small amounts can make aquifer water undrinkable for millennia. Waste from feed lots, fertilizers and pesticides and erosion from farmland, and bacteria from sewage pollute ground water. Salt spread promiscuously on icy highways finds its way into aquifers: in many Massachusetts communities people on low-sodium diets are advised to drink bottled water.

The Ogallala aquifer, with a volume equivalent to that of Lake Huron, serves hundreds of millions of farm acres in Nebraska, Kansas, Colorado, New Mexico, Oklahoma and Texas. Irrigation takes more water from the Ogallala than the entire flow of the Colorado River, but in five of the six states irrigated acreage is declining, and the aquifer may be effectively exhausted in 40 years.

An average of 4.2 trillion gallons of precipitation falls on the contiguous 48 states daily, but 92 percent evaporates immediately or runs off, unused, to oceans. More water must be captured for use and more used water must be recycled. And agriculture, which accounts for 87 percent of used water, must use it more efficiently. Today, unreasonably cheap water encourages, for example, wasteful "flood irrigation." The runoff washes into streams hundreds of thousands of tons of mineral salts that accumulate in, and damage, lands that use the water downstream.

American agriculture, the principal sustainer of the nation's standard of living, is among the most science-intensive industries. It will-- when spurred by rising prices--make extraordinary efficiencies in water use. Already some western fields are flecked with little reflectors that enable lasers to guide graders that level fields to within a 1 percent variation, thereby minimizing runoff. This and other technologies should enable water savings of 20 to 50 percent.

What is less certain is a sensible water policy from Washington, where at least 70 congressional committees are involved. Furthermore, states are fiercely protective of their traditional rights regarding water. But the doctrine of "states' rights" is nowhere more anachronistic than regarding water, and not just in the West. For example, Boston, which loses about one- third of its water through leakage from its an cient system, has aroused resistance from Canada to New York with a proposal to pump more Connecticut River water.

But the Reagan administration is chock-full of westerners blind to the connection, philosophical as well as etymological, between conservation and conservatism, properly understood. The administration is apt to defend the traditional Washington role regarding water, primarily a role of providing subsidized water for the West--a tradition dating from the days when westward migration had to be encouraged.

Out west, detestation of the federal government by "sagebrush rebels" stops well short of a desire for fewer reclamation dollars. And reverence for the market stops well short of a belief that users should pay market value for water.