Serious crime in Northern Virginia declined the first six months of this year, with Falls Church reporting the biggest decrease, 26 percent, according to figures recently released by area police departments.

The smallest decrease was in Alexandria, where serious crimes were 4 percent lower the first half of this year compared with the same period in 1981. In Arlington, the decrease was 17 percent, while in Fairfax, serious crime dropped 21 percent.

Police officials say it is difficult to pinpoint reasons for the decrease, but they say a major factor may be the increasing awareness of citizens.

"We have received many more calls from neighbors who report suspicious activity," said Arlington Police Chief William Stover.

Stover also attributed the decrease to cooperation among Northern Virginia police departments. "Police have jurisdictional boundaries," he said, "criminals do not. We've done a lot to develop close working relationships across these boundaries."

But local officials say they are only guessing about the reasons for the declines.

"Some say the cold weather earlier this year kept people off the streets. Some say the economy is bad and there is less crime because people are sticking together. Others say the opposite," said Barry Hulik, of the Arlington Police Department information office. "It is a combination of things probably. A good combination."

The sharpest decrease in all jurisdictions was in burglaries. In Arlington, 750 burglaries had been reported as of June 30, compared with 1,081 for the same period last year, or a 31 percent decline. Falls Church residents reported 51 incidents as of June 30, compared with 83 incidents for the same period last year, or a 39 percent drop. In Alexandria, burglaries fell from 1,149 for the first six months of 1981, to 896 as of June 30, a 23 percent decline. And in Fairfax County, 1,882 burglaries had been reported as of June 30, or 43 percent fewer than the 3,295 reported for the first six months of 1981.

Sgt. John T. Jones, of the Falls Church police, said the decrease may reflect an effort by many homeowners to take extra precautions when they leave their homes for work or for vacations. He said neighborhood crime prevention groups have been distributing information about how to stop mail delivery, where not to hide spare keys and other tips to thwart thieves.

"I think we are seeing more house sitters, more neighbors watching each others' homes," he said. "People are a lot more cautious."

For statistical purposes, the police departments define seven categories of serious crime: homicide, rape, aggravated assault, burglary, larceny, robbery and motor vehicle theft.

Other major changes in serious crime included:

Homicide: Arlington, up 150 percent (five cases); Alexandria, down 75 percent (two cases); Falls Church, no change (none); Fairfax, no change (three cases).

However, police said the number of homicides in Northern Virginia is low, which makes their percentages useless in determining crime trends. "For murder, the figures are so low they fluctuate widely," said Hulik. "We go from one to 10 in a given year. It is hard to average them in."

Rape: Arlington, up 41 percent (31 incidents reported); Alexandria, up 9 percent (34 incidents); Falls Church, down 25 percent (three incidents); Fairfax, down 25 percent (46 incidents).

Aggravated Assault: Arlington, no change (117 incidents); Alexandria, down 1.3 percent (145 incidents); Falls Church, down 24 percent (25 incidents); Fairfax, up 1.8 percent (164 incidents).

Larceny: Arlington, down 12 percent (2,623 incidents); Alexandria, down .1 percent (2,548 incidents); Falls Church, down 16 percent (229 incidents); Fairfax, down 13 percent (8,013 incidents).

Robbery: Arlington, down 32 percent (141 incidents); Alexandria, up 18 percent (290 incidents); Falls Church, down 38 percent (eight incidents); Fairfax, down 12 percent (241 incidents).

Motor Vehicle Theft: Arlington, down 17 percent (248 incidents); Alexandria, up 14 percent (237 incidents); Falls Church, down 52 percent (12 incidents); Fairfax, down 25 percent (605 incidents).

The police spokesmen all said crime statistics are tricky and do not always indicate criminal trends. But Stover said they prove one thing to him: "People are fed up with crime and are starting to do something about it. They are tired of it and I think they're right."