The number of rapes reported in Arlington County has increased 10 percent so far this year over last year and 58 percent over the number of rapes reported in 1979, according to Arlington police figures.

"We've been hit hard this year, especially in September and October when we had nine rapes both months," said Det. William J. Summers of Arlington's three-member sex squad. "We shouldn't be getting that many rapes."

Summers attributed the increase to several factors: The number of single women living in Arlington has risen sharply recently, with many of them living in Arlington's numerous ground-floor apartments where 17 rapes were reported this year; more women also work these days and therefore come and go from their homes at odd hours; and more women today are willing to report rape to police than only a few years ago.

As of Thursday, Summers said the police had logged 55 rapes and 19 attempted rapes, compared to 50 rapes and 15 attempted rapes through the same date last year. (Another rape was reported Friday night, but police had not yet categorized it as a rape or attempted rape. Police have closed the cases on 59 of the 74 categorized incidents, most of them rapes.) In 1979, 32 rapes were reported in Arlington.

In Alexandria and Fairfax County rapes have decreased so far this year. In Alexandria, 55 rapes were reported in the first 10 months of last year compared to 52 for the same period this year. In Fairfax County, where police had reports for only the first six months this year, there were 46 rapes compared to 61 reported last year.

In 1970, according to U.S. Census data, almost 45 percent of 76,259 women living in Arlington were single, widowed, divorced or separated. By 1980, those women accounted for nearly 58 percent of the 70,031 Arlington women.

Although the rapes are spread throughout the county, police say a majority of them have taken place in the southern part of the county, below Rte. 50. From April through September, six rapes occurred in a small area in southern Arlington. Three of the rapes took place on one block. A man was convicted of one of those rapes and police say other arrests are imminent. The rapes in the area have stopped, Summers said.

Four of those women, like 13 other victims this year, lived in ground-floor garden apartments, which tend to be targets of rapists more than single-family homes or high-rise buildings, Summers said. It is easier to enter those apartments, which often lack air-conditioning, through windows left open at night, he said.

Although many rapists study their victims to establish their daily pattern, tip-offs such as flowers in the window may signal the presence of a woman living there, Summers said. He recommended that women vary their daily routines and install good locks. Police also give neighborhood seminars on rape prevention upon request.

Some of the increase in reported rapes, said Sgt. Michael Schweitzer, the sex squad's supervisor, may result from women now being more willing to report the crime because of changing public attitudes that less often blame the victim for causing the crime.

"It used to be that you couldn't bring a rape victim near the police department because she felt humiliated and was afraid people would find out," Summers said. "One of the first questions they ask is if it will be in the papers."

He said judges and juries also have gotten tougher in sentencing convicted rapists, which also may encourage women to report rapes. An Arlington court this year sentenced one convicted rapist to 80 years in prison and another to 45 years.