Let's take an elementary principle of economic -- when demand exceeds supply, prices will rise -- and apply it to Champagne.

Demand: In 1981, 159 million bottles were shipped from the cellars of Champagne, one-third of them outside France (the United States becoming the leading importer for the first time). In 1978, 186 million bottles were shipped. Demand has not declined. Supply has.

Supply: Officially, very low. The harvest in three of the past four years has been below average. In fact, there are hundreds of millions of bottles in the chalk cellars of Champagne. But these are the reserves and, as the name implies, not intended for immediate sale. Champagne houses estimate their stock level in terms of average annual sales, and they like to keep a reserve of between four and six years' supply.

Prices: There have been frequent increases since 1978, initiated by the higher cost of grapes paid by producers to growers to offset small crops and general inflation. Prices are stable for the moment, thanks to the strong dollar and the policy of the large champagne houses, the n,egociants- manipulants, of offering favorable prices to their export customers. (Demand within France is so strong that there's no need to keep domestic prices down.) Then why keep all those valuable assets in reserve? Why not take advantage of a sellers' market? It would be too risky to deplete the reserves, say the large houses. First, nature being as unreliable as it is in this most northerly wine region of France, the reserves are insurance against a run of small harvests. Second, those reserves are gradually maturing in quality. They are lying on their yeasts, or more accurately and graphically, on their necks. The disgorging, or removal of the yeasts and other deposits of the second fermentation, is still to to come. Lengthy contact with the protein deposits enriches the wine.

Now, a house could decide to shorten the period the wine lies "en tirage." Most reputable producers double or treble the minimum requirements for aging. But in the selling of a champagne, reputation is everything. We buy a champagne by name, the name of its producer, not a vineyard or grape, and the famous houses of Epernay, Reims, Ay and Chalons-sur-Marne value their reputations too highly to cut corners.

They don't need to. All they need do is ensure that their customers have a continuous supply, no matter how tiny the trickle. And if that means that the prices are increased further along the supply line, well, so be it.

In a wondrously perfect vintage, Champagne's total permitted production capacity is 250 million bottles. It cannot possibly satisfy the world's thirst for better quality sparkling wine. No matter, say the producers, for there'll always be a market for the traditional, superior qualities of champagne. "If champagne is too easily available, it loses its prestige." Nevertheless, with reserves being below normal, I had a feeling that they would prefer to be less vulnerable than they are right now.

What do they want for 1982? A large, healthy crop and stable prices from grower to consumer. I'm sure we'll all drink to that.