If this be spring, it must be time for the annual shower of comprehensive, unequivocal, statistically significant, scientifically hard-nosed and utterly unassailable pronouncements on acid rain.

President Reagan is meeting next week with Canadian Prime Minister Brian Mulroney, and the politically sensitive issue of acid rain is high on the agenda. Congress is poised for another round of legislative battles on the subject, and the strategists in both industry and environmental groups are marshaling the latest in scientific opinion to bolster their causes.

Well-timed studies are nothing new in Washington, where statistical analyses to prove almost any point can be churned out in the twinkling of a lobbyist's eye. But over the last five years, acid rain has raised the ancient art to new levels of abstraction.

Depending on whose scientist you heed, either the United States must move quickly to curb the sulfur pollution that is damaging lakes and forests in the Northeast and Canada, or it should not move at all. Between the conflicting opinions hangs a multibillion-dollar program of pollution control, and few with any stake in the matter are willing to await the results of the Reagan administration's policy of study and more study.

So it is that the independent study season was opened this week by the National Coal Association, which summoned reporters to a briefing and broke out copies of a report called "The Downward Trend in Sulfur Dioxide Emissions at Coal-Fired Electric Utilities."

The industry said the study "unequivocally demonstrates" that sulfur pollution, believed to be a leading cause of acid rain, is on the wane despite increased use of coal. (Point: No need to take action against coal-burning power plants. The problem is solving itself.)

The coal industry's effort was quickly followed with "Acid Deposition and the West: A Scientific Assessment," a slickly packaged report commissioned by the Western Energy Supply and Transmission Associates, an organization of western utilities.

The study was written as a rejoinder to a study done last year by the Washington-based World Resources Institute, which concluded that acid rain was imperiling national parks. (Point: The problem is national in scope and pollution-cutting in the East alone won't fix it.)

Not surprisingly, the utilities' study, heralded as a review of "the most up-to-date scientific work," found "no credible scientific evidence" that acidic pollution was having any widespread effect in the West. (Point: Do what you will in the East, but leave us alone.)

This is what one congressional aide called "the Battle of the Bilge," an engagement of data points and statistical averages, historical norms and computer extrapolations.

Credentials are armor in this kind of sortie, which may be captained by lobbyists but is generaled by the guy with the five-star doctorate.

The western utilities' study, for example, was produced by six PhDs under George M. Hidy, director of the Desert Research Center at the University of Nevada-Reno. Hidy also serves as a scientific adviser to the Environmental Protection Agency and served as a member of the review panel for the U.S. and Canadian acid-rain research effort.

Hidy said he sees no conflict in the dual roles, even though his utility study took issue with one of the key findings of the U.S.-Canadian effort -- that natural sources of acidity did not contribute significantly to the acid-rain problem.

So where's a policy-maker to turn? Well, there's always the National Academy of Sciences, often touted as the be-all and end-all of scientific judgment. And as luck would have it, the NAS has a study coming out this week, just in time for weekend browsing before the Reagan-Mulroney summit.

The timing, according to an NAS spokesman, "is simply due to the fact that it came from the printer today."