Faced with the prospect of a severe summer drought, governors of four rain-thirsty northeast states today declared a "water emergency" for the 22 million people who live in the so-called Delaware River basin. The declaration bans all nonessential uses of water.

But the governors, who met in Trenton, stopped short of drastic actions such as rationing, in the hope that the formal emergency declaration would convince water users of the need to reduce consumption to forestall a water shortage this summer, when demand for water will increase sharply.

The governors, Hugh Carey of New York, Brendan Byrne of New Jersey, Pierre DuPont of Delaware and Richard Thornburgh of Pennsylvania, presumably are seeking a psychological, rather than measurable, impact. Most of the nonessential uses covered by the ban are warm-weather activities, such as car-washing and lawn-watering. Penalties for violating the restrictions range from $50 to $1,000.

To lend authority to the declaration, the governors attended the meeting of the Delaware River Basin Commission personally, instead of sending delegates.

The four governors also ordered New York City to cut 40 million gallons from the 560 million gallons it draws each day from its huge reservoir system at the headquarters of the Delaware River. The emergency ban does not apply to the city even though it relies heavily on the Delaware River for its needs. The city itself is not in the river basin. Until last October, when the lack of rainfall began to reduce the flow in the Delaware, the city took 800 million gallons a day from the river.

New York Mayor Edward Koch confirmed that he plans to declare a water emergency in New York City next week. The New York plan, like the governors', at first will rely mainly on voluntary measures and a public campaign to reduce water usage. Koch appears in a series of TV commercials, in which 43 schoolchildren admonish their parents not to waste water. Since November, New York restaurants have served drinking water only on request.

Since last June, rainfall has averaged about 30 percent below normal in the Delaware basin, according to Harold Gibson, meteorologist in charge of the National Weather Service's New York office. Gibson said dry weather is likely to continue for the foreseeable future.

Normally at this time of year New York's reservoirs are 80 percent full.Today, they are at less that 33 percent of capacity, at best about a 100-day supply of water. City officials want to reduce daily usage from 1.55 billion gallons in November to 1.25 billion gallons a day. During the summer, New York City has consumed as much as 2.3 billion gallons a day, and regularly averaged 1.7 billion to 1.9 billion gallons.

New York City, Trenton and Philadelphia are among the major metropolitan areas that rely on the Delaware for all or a major portion of their water. The drought was also hit large segments of New Jersey that do not use the Delaware River. Nearly 170 suburban New Jersey communities have rationed water since last September.

Carroll Saboe, of the U.S. Geological Survey, said that at Trenton 3,788 cubic feet of water now flow each second, compared with 11,670 last year.

The governors and a federal representative on the five-member commission are worried not only that the low flow in the Delaware will cause water shortages, but also that the river will not flow fast enought to cleanse itself of pollutants. Near Philadelphia there is concern that ocean water will begin to move upstream and contaminate that city's water supply with salt.