The Arlington County Board has voted to put a clarifying referendum question on the November ballot asking voters to spend $6.1 million in previously approved bond money on improvements to the county water system.

In 1977, Arlington voters approved the bond issue for construction of new water storage facilities. At a meeting last week, however, the County Board decided it was more important to improve the existing system. Under state law, the county must get voter approval to use the money for a different purpose that originally planned.

At the same time, the board defeated a proposal to put another bond issue, for $6.2 million to build additional storage space facilities, on the November ballot. That proposal was defeated after several members of a citizens' committee urged the board to delay the referendum, saying that two similar issues on the same ballot would confuse voters.

"I think we're vulnerable until we get more water storage," said Ellen M. Bozman, the only board member to support placing both questions on the November ballot.

If the first question is approved, engineers will clean and replace several corroded pipes and water mains which cross the Potomac River from the Dalecarlia Reservoir in the District.

In other action, the board approved an increase in rates charged by the six taxi cab firms licensed in Arlington.

Under the new rates, which take effect within 30 days, a five mile trip will cost $5.30, a 50-cent increase over the old rate.