All of us know a calendar year. Those who work for, or deal with, the government know about a fiscal year. Yesterday, we discovered yet another annual cycle, the water year.

The U.S. Geological Survey, sending along its monthly bulletin on the fresh-water flow into Chesapeake Bay, summarized information about "the water year, which ran from Oct. 1, 1983, through Sept. 30, 1984."

The flow into the Chesapeake during the water year, we should be pleased to know, was the third highest on record for the entire bay dating back to 1950. The flow of our hometown river, the Potomac, was two-thirds more than the long-term average. More on that later.

Okay, so back to the three kinds of years. The calendar year is one we all know. The government's fiscal year for a long time ran from July 1 through June 30, but as Congress had increasing trouble passing departmental appropriations bills, it got pushed back to Oct. 1 through Sept. 30.

That, it turns out, is also the traditional water year recognized by hydrologists. Those specialists, the Geological Survey reports, consider it an honor that the government has adopted the water year as its current fiscal year.

Back to the water.

During September, the Potomac's average flow was 2.4 billion gallons daily, one-third above the long-term September average.

During the water year, daily flow averaged 12.3 billion gallons, the ninth highest in 54 years of Potomac River record-keeping. The range was startling: a low of 1.3 billion gallons last Oct. 11, to a high of 132 billion gallons on Feb. 16.