If you have access to a TV set or radio, Gardon Barnes' weather forecasts are almost as free as an hour of sunshine.

But let us say you want to know, badly, if it's likely to rain in your backyard the second week of September. Gordon Barnes will tell you, promising 85 per cent accuracy - but the information then is not free.

With its Occoquan Reservoir falling to record low levels every day, the Fairfax County Water Authority has decided to hire Barnes for just such tailor-made long-range forecasting. The authority wants to know if its reservoir - the water supply for 600,000 Northern Virginians - is likely to become a dust bowl in the remaining 2 1/2 months of the dry season.

For $1,700, Barners, who runs a private forecasting service, will provide the authority with detailed rain forecasting for September and October.Using such data, the agency hopes to be able to make well-timed recommendations on when Northern Virginia jurisdictions should intsnsify current water emergency measures.

While the National Weather Service provides 30-day forecasts, they are too general for the water authority's purposes, and besides the predictions cover a region much broader than the Occoquan watershed, which extends through Fauquier, Prince William and Fairfax countries.

Under new emergency criteria adopted along with Barnes' budget Thursday night, the authority says it will recommend stiffer measures when the reservoir falls to certain levels. For example, the criteria say that when the reservour's level goes down to 92 feet (it is now at 105 feet, 4 inches), certain businesses that use a lot of water would be closed.

As authority spokesman James Warfield said, if Barnes says a heavy rain can be expected just as the level goes down to 92 feet, tha authority might hold off recommending th closing of businesses.

For his long-range forecasts, Barnes looks at records going back to about 1890, when meterological records first began to be compiled in a systematic way, and also a mass of current computations on air movements from west to east.

The computations, done by Irving P. Krick Association in Palm Springs, Calif., are based on a theory developed at California Institute of Technology 40 years ago that said weather moves across the country from west to east in six-day cycles.

With all the historical records and current data in hand, Barners,starting next week, will forecast "how much and when it will rain" in the Occoquan watershed.

Barnes said the clients for his service are priliminary agriculture-related, such as food processors who want to know what kind of yield they can expect and when to go to market for the most favourable price.

He said his associate, Harry Geise, was recently hired by Marvin county, the water-short area near San Francisco that has had two years of drought.

Under the authority's newly adopted criteria, mandatory water conservation (essentially limiting out-door residential use) would be imposed if the reservoir falls another 3 feer, 2 inches. The reservoir recently has been dropping two to three inches a day.

After water consumption fell to about 54.5 million galloons daily earlier this week, it rose Thrusday to 57.2 million galloons. Northern Virginia officials renewed their plea for continued voluntary conservation.