With water reservoirs at dangerously low levels, government authorities say portions of the East Coast face what could become the region's worst water shortage.

The situation, which Pennsylvania Gov. Richard L. Thornburgh has called "the most critical water shortage in the Delaware River Basin in this century," has resulted in proclamation of a drought emergency.

Mandatory restrictions on water use have been imposed in New York City and neighboring Westchester County, in 16 counties in eastern Pennsylvania and in 93 communities in northeastern New Jersey. Connecticut says it is monitoring water levels carefully.

"If it doesn't rain in the next four or five months, the drought has the potential of either meeting the worst drought on record -- in 1965 -- or even beating it," said Andrew McCarthy, a spokesman for the New York City Department of Environmental Protection.

The city's three reservoirs, which have a capacity of 550 billion gallons, are only 61 percent full. Or, as Mayor Edward I. Koch said Friday when he proclaimed the second drought emergency in four years, "We are missing 150 days worth of water." Koch challenged New Yorkers to conserve 100 million gallons daily by taking brief showers rather than baths and urged them to keep a watchful eye on running faucets. He also announced measures that require commercial and industrial users to cut water consumption by 15 percent.

Restaurants are allowed to serve water only upon request, and washing of cars and paved surfaces is prohibited. So are opening fire hydrants without a permit and using ornamental fountains. Watering lawns and golf courses is limited to early morning or late evening. Violations are punishable by fines ranging from $100 to $1,000.

Emergency measures remained in effect in the city for an entire year during the last drought, in 1981. If this year's situation does not improve soon, authorities said, fines may escalate and even more intense conservation may be required of commercial users.

As a last resort, the city is prepared to pump water from the Hudson River, as was done in the 1965 drought, the region's worst.

In Pennsylvania, where rainfall is 11 to 16 inches below normal, the restrictions are similar to those in New York City and provide for a maximum $500 fine and up to 90 days in jail for violators. Irrigation of gardens, trees and shrubs is also restricted.

"We are telling people that they should not expect the emergency to end before fall, at the very earliest," said Michael Moyle, a spokesman for Thornburgh.

Moyle noted that Pennsylvania needs in excess of 5 inches of rain monthly, an inch more than normal, over the next four months just to prevent the situation from worsening.

New Jersey's reservoirs have fallen to 60.2 percent of capacity, and officials said last week that more severe restrictions, possibly even rationing, may have to be imposed, perhaps within a week.

"We anticipate asking for voluntary conservation, or we may impose rationing," said James Staples of the New Jersey Department of Environmental Protection.