A volunteer citizen group in Fairfax County is interested in finding out about horse power. But it's not the kind of horse power that you find in cars. The group is involved in a survey of horses in the county and wants horses, and people who ride horses, to get a share of the money the county will spend in the next four years on planned multi-purpose trails.

Since July, Leslie Clark and 75 volunteers have distributed 1,500 questionnaires to horse owners in the magisterial districts of Centreville, Dranesville and Springfield. More than 415 questionnaires have been returned.

Clark will present survey findings to county park authority members at an informational hearing Tuesday at 8 p.m. at park authority headquarters, 4030 Hummer Rd., Annandale.

"I see this hearing as a fine opportunity to present the equestrian case for trails and facilities in a reasoned, factual manner," Clark said.

The county has $2 million to spend for the trails under its Capital Improvements Program. The Northern Virginia Regional Park Authority have other money for trail and park expenditures, and equestrians want to make sure they're considered in all plans.

Volunteers worked with the Environmental and Technical Services Branch of the county's Comprehensive Planning Office and the office's trails coordinator, Barbara Halpern. Joseph Hayward (Skip) todd III, a county extension agent, helped design the survey and his office receives completed questionnaires.

In July, 1976, the county Board of Supervisors adopted a plan to establish trails for transportation and recreation purposes. Before the adoption planners asked citizens what kinds of trails they wanted. Each magisterial district has a Citizens' Task Group responsible for fostering trails and parks. In Dranesville, which includes McLean and Great Falls, and in Centreville, which includes Vienna, Oakton and Chantilly, citizens said they wanted horse trails. But they didn't do a great deal to make their wishes heard.

"Horse people don't organize easily," said Clark, who lives near Vienna with her husband, two children and two horses. "Some think Fairfax County may have the largest horse population on the East Coast. Our figures indicate conservatively 4,650 horses in three of the eight magisterial districts. But horse owners haven't made their presense felt in an effective, positive way."

Todd, in his extension office, had been pondering the horse world as a result of his work with 4-H Clubs. He had tried to garner information by leaving a fairly simple questionnaire in various tack shops - stores which sells boots and bridles, saddles and other gear.

Clark found his questionnaire in such a shop. She assembled volunteers, and with their help and Todd's, the group redid and field-tested the questionnaire. The voluntters then set out to put a copy into the hands of all horse owners in Dranesville, Centreville and Springfield districts, which were thought to have the most horses.

On foot and from their cars volunteers looked for horses. Seeing one or more in a field, they sought out the house to leave a questionnaire, preferably with a member of the family. Recipients could mail it back to Todd's office in a U.S. Department of Agriculture franked envelope.

Those responding gave only their zip codes; surveys were color-coded by magisterial district for tabulation purposes.

Ninety-six per cent of those answering have said they want their sport included in county trail and park plans. They don't insist on expensively surfaced trails - just dirt. In fact, hard surfaces after a time can ruin the feet and legs of a trotting or cantering horse. They say they would use the trails two to three times a week, but they want their presence on the land to be legal, and they want a trail to have a destination.

They also want connector routes between trails which are open to horses. Present county plans call for such routes alongside main highways such as Routes 7 and 123. These would be open to hikers and bicyclists as well as riders.

Of the owners responding to the survey, 67 per cent see a need for some kind of trail policing. And 82 per cent called for a volunteer agency, to be assisted by the county, to clear and patrol trails, sponsor trail programs and provide legal information about easements across private land.