The Occoquan Reservoir, which had fallen almost every day since late August, received about 200 million gallons of water - or little more than a four days' supply - from the weekend's long, steady one-inch rain.

If more rain comes Wednesday night and Thursday morning - as predicted by WTOP forecaster Gordan Barnes a consultant to the Fairfax County Water Authority - the agency may postpone buying emergency supplies from the city of Manassas. The authority said last week that barring significant rainfall. It would start on Wednesday purchasing about 43 million gallons daily toward a maximum 1 1/2 billion gallons.

About a quarter inch of rain has been predicted by Barnes for Wednesday-Thursday. Although that is only a quarter of the amount that fell on Saturday and Sunday, it could produce more runoff into the reservoir because the soil is now wetter. The quarter inch. authority spokesman James A. Warfield Jr. said could produce an additional 250 million gallons of water.

Under the authority's guidelines, it would start purchasing water from Manassas when the reservoir falls to the 94 1/2 foot mark (above sea level). The reservoir was at 96 1/2 feet yesterday.

If the authority buys all the 1 1/2 billion gallons it has contrated for the cost would be about $172,500. The purchase price per thousand gallonse decreases as more water is bought.

According to Barnes' long range forecast. October rainfall should be above normal. How much of that rain would be converted into runoff, instead of being absrobed into the ground to raise the depleted water table, is authority officials say.