A drought warning urging 22 million people in fours states to begin conserving water was issued yesterday by the Delaware River Basin Commission because dwindling supplies have left reservoirs only one-third filled.

Water levels in the region are "substantially below normal as a result of rainfall deficiences . . . and severe drought conditions could develop in the spring and summer of 1981 if these trends continue," said a resolution unanimously approved by commissioners from Pennsylvania, New Jersey, Delaware, New York and the federal government.

Below-normal rainfall is forecast for the region this winter.

Among more than a dozen witnesses at the afternoon-long hearing was Gretchen Lehy, secretary of the Pollution Control Group of Lower Bucks County, Pa., who charged that the commission has "deceived and misled" the people of Delaware River Basin by placing the basin's interests below those of New York City.

"New York City will bleed us dry and you do nothing," she charged. "We are deeply involved in a full-fledged water war with New York City."

There was no immediate reaction from officials of New York City, which earlier this fall piped water from its reservoir system to drought-stricken northeaster New Jersey.

Also critical of New York City usage of Delaware River water was Val Seigstedt, Point Pleasant, Pa., who said "We must get to the conscience of New York City, else we have to go to war with the city. This commission has put a drought-prone river in serious danger by giving it away to New York City."