Rep. Herbert E. Harris II (D-Va.) charged yesterday that last summer's water shortage in northern Virginia resulted from mismanagement of available water rather than drought conditions.

Harris, chairman of the House Disctrict Subcommittee on Regional Affairs, said an investigation by his congressional staff showed that 600,000 suburbanites were unnecessarily prohibited from watering their lawns and washing their cars because of bungling by local officials.

Harris held a hearing on the issue yesterday that had political overtones as deep as the Occoquan Reservoir after a heavy rain. Harris directed much of the blame for the water shortage at a potential challenger for this congressional seat. Fairfax Board of Supervisors Chairman John F. Herrity.

Republican Herrity, who nearby drowned in a sea of statistics unleashed by Harris, denounced the hearing as "a political show."

Harris charged that the Fairfax County Water Authority, "with no more than the turning of a valve," could have averted the shortage by purchasing water from Fall's Church, which abuts part of the county's eastern end.

The main source of water for Fairfax County is the Occoquan Reservoir, a huge impoundment on the Fairfax. Prince William County border, which collects water from runoff over a wide area of northern Virginia. Falls Church buys its water from the Army Corps of Engineers, which pumps water from the Potomac River.

Harris said it wasn't until Sept. 20, three days after mandatory conservation controls were imposed, that the Fairfax water authority purchased the maximum amount of water available from Falls Church.

The charges, accompanied by supporting testimony from the Corps of Engineers and Falls Church officials, appeared to stun Herrity, who said the Board of Supervisors was not aware that additional water was available from Falls Church last summer.

Fred C. Morin, chairman of the Fairfax County Water Authority, said last night that "we were getting all the water we could" from Falls Church last summer. He said additional water might have been available early in the summer, "but we didn't have a problem until August. Then it took some time to make the arrangements."

Herrity said Harris was "all wet . . . about the so-called Falls Church connection." he accused Harris of staging a "political show." If Harris were really interested in getting the facts, Herrity said, the member of Congress would have shared his findings with members of the water authority in advance so they could have responded to the charges at the hearing. Instead, "he pulled his charges out of a hat," Herrity said.

Herrity's testimony before the subcommittee consisted of replying to a series of written questions posed in advance by Harris.

As Herrity explained how the decision was made to ask first for voluntary cuts in water use, and then mandatory controls, Harris interrupted to unload the first of his findings, then mandatory controls, Harris interrupted to unload the first of his findings.

Rainfall between last April and August totaled 16.35 inches, compared to 14.71 inches for the same period the year before, he recited.

"There doesn't appear to be a drought from those statistics," said Harris, flashing a boyish grin.

Herrity replied that "you can't calcualate" water supply by measuring rainfall. "It's not what comes down." Herrity explained, but how much runs off into the reservoir. Because of the severe winter, much of the rain was absorbed into the ground because of a low water table, he said.

Harris conceded the point, but then unloaded his big charge, complete with oversized charts that showed that Fairfax had passed up 987 million gallons of water available from Falls Church.

"Nearly a billion gallons," Harris said, repeating the charge. Herrity interrupted and accused Harris of playing "a numbers game - Monday morning quarterbacking."

Harris directed a pointer at the lavender, blue and red bar graphs on the 5 by 8-foot charts, and as a teacher would explain to a student, told Herrity, "Let me show you Jack."

Herrity tried to respond, but Harris, his voice rising in anger, dropped the informality and shouted, "Excuse me, let me finish, Mr. Herrity."

The leadoff witness was Secretary of the Army Clifford L. Alexander Jr., who praised Harris for his "continued support and interest" in Washington area water problems. Alexander was flanked by a general and colonel from the Corps of Engineer who testified that water from the Potomac was available last summer through Falls Church for Fairfax County.

D.C. Del. Walter E. Fauntroy, a fellow Democrat, also supported Harris' contentions, saying District residents would be alarmed if they were forced to conserve water because suburbanites in Virginia has squandered their fair share.

Harris said his motives for the bearing were nonpolitical. He said he just wanted to make sure the same situation doesn't occur again this summer.

Fairfax County Supervisor Alan H. Magazine D-Matson) said last night he has asked County Executive Leonard Whorton to investigate the charges posed by Harris, whom he served with on the Fairfax Board.