The level of the Occoquan Reservoir, whose three-month decline triggered a voluntary water conservation program in Northern Virginia, has risen more than a foot as a result of Wednesday's heavy rain.

The rain added about 350 million gallons to the reservoir - a little more than six days' supply. But Fairfax County Water Authority officials noted that the reservoir with less than 4 billion gallons of water still held far below its normal quantity of 7.5 billion to 9.8 billion gallons. The authority urged its customers to continue to restrict outdoor and other usage.

Consumption, which ordinarily runs 60 million to 65 million gallons daily, totaled 54 million gallons on Thursday, an improvement over the previous three days, when usage averaged a little more than 58 million gallons.

While the Occoquan was inching upward, so was the Standard & Poor's Corp's bond rating for the authority - from BBB+ to A. According to Fred C. Morin, chairman of the authority, the higher rating was the result of the agency's "sustained good management and operating performance" and improved financial prospects as a result of the increase in customer rates that took effect Aug. 1.

Morin said the improved rating will permit the authority to obtain lower interest rates on future bond issues and will widen the investor market for its bonds.