THE KENNEDY CENTER, with a powerful assist from the Rockefelle, Foundation, has just concluded arrangements that are likely to produce annually one of the year's most exciting musical weeks here. On Wednesday, Center and foundation officials announced the creation of the John F. Kennedy Center Rockefeller Foundation International Competitions for Excellence in the Performance of American Music.

With annual funding foundation funding of up to $200,000, the competitions will vary from year to year in the area of musical activity. THe first contest, to be held at the Center the week of Sept. 11 next year, will be for pianist - pianists of any age, sex ad nationality. Future competitions will be open to singers, small instrumental combinations and possibly solo instruments other than the piano.

he distinguishing element among the requirements of this competition, one that completely sets it apart from other major contests in this country and Europe - which it matches in artistic standards and financial rewards - is this rule:

"More than half of each program in the finals) must be made up of American music, with the option of choosing the remainder from works in the standard and contemporary repertoire."

Somee of the reasoning behind the establishment of the new competitions was stated in the latest Rockefeller Foundation report, an informal publication that comes out every three or four months. "Not one piece in the standard repertoire, for example, has been composed by an American," says a brief article by Joseph Roddy.

Had the Rockefeller report wanted to carry its observations a step farther. It could have said, with a distressing degree of truth, that most young American pianists today are likely to know perhaps one major work by an American composer. As for foreign pianists - and the new competitions are international - most could not even name an American composer other than George Gershwin, Leonard Bernstein or possibly Aaron Copland.

For an example of the importance attached to the piano music written in this country in this century. England's forthcoming Leeds International Piano Competition, which has become one of the larger contests in recent years, Lists pages of repertoire from which contestants may choose. In those pages, there is all of one work by an Americn Copland's Piano Variations.

The most cursory look at the list of suggested works published by the Center-Foundation planners makes you wonder how and why it has been so long since major pianists played Henry Cowell's piano music, or a sonata by Norman Dello Joio, or Leon Kirchner, or Vincent Persichetti, or George Rochberg or Roger Sesions.

(There is a certain irony to all this in Washington, where, for many years, some of our finest pianists like Evelyn Swarthout, Margaret Tolson, Roy Johnson and Robert Parris have consistently played these and many other American works. But they have nearly always played them for small audiences who did not pay admission to hear them.)

If this body of music comes to American pianists with any sense of novelty, it will be even more of a surprise to European pianists, most of whom have grown up in total darkness as far as transaltantic music is concerned. Yet there have been times, as in the course of various Cliburn Competitions in required work by an American composer among its challenges, when European visitors have surpassed Americans in the insights and brilliance with which they have played our native music.

Is there any hint of chauvinism in this new competition? I think none at all. It is not as if there were the slightest need for apology about the vast body of American piano music that is eligible for this contest. From the sonatas of Charles Ives and Edward MacDowell to the Partita Variations by George Rochberg, from the Sonata Teutonica by John Powell, recently edited, published and magnificently played by Roy Johnson, to the Nostalgic Waltzes of Ross Lee Finney or the new Etudes by John Corigliano, there is a breadth of style and idiom, of sonorities and imagination to be found in this country's piano music that can stand comfortably beside that which has come from Europe in the same years.

The new contests have been skillfully designed to lure the world's finest pianists to the Kennedy Center. Prize money reaches $30,000, to which are added, for the top winner, a concert tour of this country, limited recording possibilities, and if the winner chooses to play a second season of programs with the same proportion of American music, an added $5,000 bonus.

The impact of this new idea in competitions on our concert life is further heightened by an added $35,000 that will be available for concert managers who engage the young pianists whose programs will include these major American works. Why money to the managers? "Just in case of loss at the box office," where the American public often does not put down its money when American music is being played.

The second competition - in 1979 - in which singers from all over the world will sing songs by American composers, could be the spark that relights the fire of song recitals of the kind that used to fill our halls before the opera nuts and their idols took over. (And managers caved in on that front.)

Chamber music will follow in a year, according to present plans, and that might even persuade the ensembles that play at the Library of Congress that there is a great repertoire of American music to be heard there every week, instead of now and then.

Competitions have become a way of life for thousands of young musicians around the world. It takes so much time for contestants to prepare the lengthy repertoire specified for each one; there are musicians who perform well under the tensions of contests, and musicians who do miserably; there are judges who are biased against some kinds of playing, and other judges who dislike major portions of the required repertoire; often winners fail to ignite the public fancy, while some who place second, third, fourth, or not at all, take off and rise steadily to the top. All these things are true.

The new Kennedy Center-Rockefeller Foundation Competitions, with their double focus on American music as well as on excellence in general, have a whole new reason for being, and a new reason for our celebrating their institution. To reassure any who may wonder, every pianist who enters the competition will be prepared to play any work of his choice out of the whole world of piano music from Mozart to Prokofiev, along with the American music. The two finalists will, indeed, win with "the excellence and effectiveness of their total program."