In an effort to help halt the decline of the water level in its Occoquan Reservoir, the Fairfax County Water Authority is almost doubling the amount of water it purchases wholesale from Falls Church.

Yesterday the authority bought almost 9 million gallons from the city. Normally the agency buys 5 to 6 million gallons daily. Under a 10-year-old agreement, the authority can purchase up to 10 million gallons daily.

Voluntary conservation among the authority's 612,000 Northern Virginia customers in Prince William and Fairfax counties and Alexandria has kept consumption down to about 55 million gallons daily, a reduction of more than 10 per cent. But because of a lack of rainfall, the reservoir dropped to a new record low - 105 feet, 10 inches. At that level, the reservoir contains fewer than 4 billion gallons - about half of the amount that would be normal for this time of year.

Although it, too, has been affected by drought, the Potomac River has so far been able to meet the water demands of the 2 million persons that it serves. The river supplies the District , more than half of suburban Maryland and part of Northern Virginia.

The flow of the Potomac yesterday was 1.2 billion gallons, according to the U.S. Geological Survey's Towson, Md., station - more than enough to meet demand, which was about one-third of the supply. In years of more severe drought, the river's flow has dropped to below 400 million gallons daily, although not on days when demand was at the level.