Bountiful rains in May left the Potomac River and the surrounding ground water table at record high levels, creating healthy if limited reserves for the Washington area in the event of a rain shortage this summer, the U.S. Geological Survey reported yesterday.

According to Geological Survey figures, the Potomac River flow averaged about 20.3 billion gallons a day during May, the highest average for any May on record, topping the old record of 18.2 billion gallons a day set in May 1932. Normal daily flow for May is about 8.6 billion gallons.

Area ground water levels also reached record highs, according to survey hydrologist Myron Lys. A local index well showed the the water table to be 9.6 feet below the land surface, almost three feet above the long-term average for this time of year and .1 feet above the previous record set in May 1973, Lys said.

This time last year, the Potomac fell sharply to an average daily flow of 3.2 billion gallons, survey records show, and the water table stood at 13.1 feet.

More important than the river flow is the ground water level, survey hydrologists say. Melting snows from winter and soaking rains in the spring "recharge" the ground water, "and if it starts getting dry in the summer, that high ground water will start feeding the river," survey spokesman Frank Forrester said.

The District of Columbia and parts of suburban Maryland and Virginia depend on water drawn from the Potomac River. In periods of summer drought, the flow at times falls below 1 billion gallons a day. Diversions of water from the river for municipal and suburban use are also at a maximum in the summer, sometimes exceeding 400 million gallons a day and nearing the total available flow.

Precipitation so far this year has totaled almost 19 inches, including more than five inches in May. The total for the first five months of the year was well above the normal of 14.94 inches, contrasted with last year when only 8.72 inches fell in the first five months, according to National Weather Service records.