Rep. Herbert E. Harris II (D-Va.) announced yesterday that he and the gether a compromise agreement to Army Corps of Engineers have put to help solve Northern Virginia's water shortage. The corps, however, said it wasn't certain that such a solution had actually been reached.

Harris told a Capitol Hill press conference that the corps had agreed to permit the Fairfax County Water Authority to remove water from the Potomac River except in periods of extreme drought, to supplement Northern Virginia's water supply. "There is no reason this permit (from the corps to the authority) should not be able to be issued in 45 days," he said.

Harris said the corps had agreed to give the authority a conditional permit to tap the Potomac River if the authority agreed not to remove water when the river is low. The corps has held up a permit to allow Northern Virginia to tap the Potomac for fear the Fairfax diversion would not leave enought water in the river to supply existing customers in the District of Columbia and Maryland.

While Harris was expressing optimism, the corps was raising doubts, and John F. Herrity, chairman of the Fairfax County Board of Supervisors, was denouncing corps officials as "politicians in uniform" who "are playing games and have a conflict of interest."

Corps spokesman M. R. Stevens at the district office in Baltimore said district engineer Col. G. K. Withers had agreed to grant a conditional permit only if there was no demand for a public hearing and no "significant opposition."

But, said Stevens, speaking in Withers' absence, various individuals and groups have raised objections and demanded a public hearing. AMong the groups, he said, were the Audubon Society of the Central Atlantic States, the Canoe Cruisers Association of Washington D.C., and the Virginia Historical Landmarks Commission.

The Virginia State Water Control Board has acknowledged that if a pipe is extended into the Potomac to provide a new water source for Northern Virginia, there would be an environmental degradation. But the board said benefits - an augmented water supply - outweigh drawbacks.

Stevens of the corps said it was doubtful that a public hearing could be held and the objections evaluated in 45 days, as Harris said was possible.

Herrity, who spend most of the day preoccupied with sewer problems, raid the water authority, which supplies most of Fairfax, all of Alexandria and half of Prince William County, should be entitled to take water out of the Potomac at high and low flow both, but added, "We'll have to take whatever crumbs the corps will drop off the table."

Even if the water authority gets a conditional permit to tap the potomac, it won't be able to do so earlier thatn the summer of 1980. Construction of the intake pipe and treatment plant has been delayed because of the impasse over the permit that must be supplied by the corps.

Up to now, the corps has declined to grant a permit because Virginia signed an agreement with Maryland allocation river water. That agreement would shrink the District's present share as the suburbs became more populous over the years.

The corps is in charge of providing the District's water. It also has broad authority over any construction - such as an intake pipe - in the Potomac, or any other navigable river.

Most of Norther Virginia's water comes from the Occoquan Reservoirs, which is operated by the waterr authority. The reservoir now is extremely law, Fairfax, Alexandria and Prince William have imposed mandatory water restrictions to keep the reservoir from going dry.

Harris said the Potomac River is low so infrequently that it should not be an impediment to a conditional permit.

The U.S. Geological Survey said yesterday that the Potomac River had its lowest flow since August, 1966. The September flow of the river is 42 per cent below normal.