THE INGREDIENTS for a new federal policy on the American West seem to be all lined up. The Census Bureau reports a dramatic increase over the past decade in the population of every state west of the Great Plains. A presidential commission gets ready to recommend that the government follow this trend and encourage more migration to the South and Southwest. The government owns most of the land and natural resources in those western and southwestern states where growth has been most rapid. And -- perfectly -- the man selected by President-elect Reagan to preside over much of that land, James Watt, is a proponent of developing and using it. What a happy set of coincidences.

Except it doesn't work out so easily. There are natural limits -- most imposed by water needs -- on the population and industry the western and southwestern states can absorb. Some of these limits have already been exceeded. Others will be if migration of people and industry continues at its present pace. Exploitation of the oil shale in Colorado, for example, will foreclose other kinds of development not only in Colorado but also in nearby states because of the demands an oil shale industry will make on the western slope's water supply. Similarly, increased industrial use of water in other parts of the West may reduce the water table sufficiently to conflict with agriculture and even cattle grazing.

It is to related questions like these that the Senate might ask Mr. Watt to speak during his confirmation hearing today. It is one thing to be in favor, as Mr. Watt is, of increasing the use made of those vast lands in the West still owned by the federal government. It is something else to have some idea of what those development policies will do in the long run to the land, and to the people who live in the region.

It may be that a new western lands policy is in the making. The Bureau of Land Management may have become too strick in its grazing restrictions. The pressures of the energy shortage may compel quick development of the West's oil, gas and coal. That population explosion, if it continues, may force a restructuring of traditional water rights.

But if there is to be a new western policy, if the clock is to be turned back -- as some westerners seem to want it to be -- to the halcyon days of the 1880s, Congress is where decisions will have to be made. And before it, or any secretary of the interior, authorizes sweeping changes in the way the federal lands are used, the long-range implications need to be much better understood than they are now. At the heart of almost every proposed change is the one natural resource the West and Southwest do not have in abundance: water. Yet the subject of scarce water seems invariably to evaporate when the political talk turns to how a less restrictive federal policy in the West would ease the energy shortage, provide an economic stimulus and unleash the potential of some of the nation's still vacant land.Mr. Watt should be given an opportunity to explain how his plans for the Interior Department and the West take into account that area's most serious and most intractable limitation.