The Fairfax Board of Supervisors yesterday delayed the imposition of mandatory water restrictions until Alexandria and Prince William County act today on the water shortage caused by the steadily declining level of water in the Occoquan Reservoir.

The restrictions, which the supervisors had voted earlier to put into effect yesterday will be delayed at least until Wednesday. The supervisors have asked that Alexandria and Prince William County impose mandatory restrictions because they draw water from the same reservoir.

Fairfax officials want to avoid the touchy political situation that might ensue from Fairfax citizens being required to conserve water while other customers of the same water source are not.

The restrictions, would sharply curtail outdoor uses. They would be enforced by the police, who could issue a misdemeanor summons to a violator.

The reservoir fell to another historic low yesterday. It now contains about 3.3 billion gallons, according to Fairfax environmental planner John H. Thillmann. This is the level at which the Fairfax County Water Authority has recommended mandatory conservation.

The authority, which owns the reservoir, supplies water to about 600,000 people in Fairfax, Alexandria and Prince William.

While all three jurisdictions have pledged to follow a coordinated course in heading off a water shortage, that goal has been threatened by regional squabbling. Alexandria and Prince William officials, for example, were annoyed last week that Fairfax decided provisionally to go from voluntary to mandatory conservation. Prince William board chairman Alice Humphries complained at the time that Fairfax wasn't following action criteria developed by the Fairfax County Water Authority.

To smooth over irritations, the Fairfax board, in its vote yesterday, said the county wouldn't petition Gov. Mills E. Godwin for the necessary emergency authority until Alexandria and Prince William had a chance to act.

At the same time, the Fairfax supervisors served notice they were prepared to hold a special meeting on the issue Wednesday if the other two jurisdictions didn't impose restrictions. The supervisors were saying, in effect: Fairfax will move together with its neighbors or, if necessary, alone.

The restrictions approved by Fairfax would prohibit outdoor residential watering, except from a container not exceeding three gallons. It also would forbid:

Operation of car- and truck-washing facilities that don't recycle water.

Washing of outdoor commercial or residential areas, except from a three-gallon can, unless cleanup is required to eliminate hazards.

Operation of ornamental fountains.

Filling swimming pools, except small wading ones requiring not more than five gallons.

Serving of drinking water in restaurants, unless requested.

The restrictions also prohibit adding water to pools already filled or supplying water-cooled air conditioners (used in some large multibuilding facilities) when the temperature is less than 78 degrees.