Stop griping! That rain yesterday was good for our gardens and our water supply. But even prior to the day's rainfall, the government tells us, things were in pretty good shape.

After six months of below-average flow of the Potomac River, its flow increased during March to 6 percent above average for the month. But, perhaps more significantly, the area's ground-water level--most meaningful to agricultural growers--rose above average for the first time in 29 months.

The ground-water rise reversed a below-average trend that began in October 1980, according to the Interior Department's U.S. Geological Survey. Its so-called "key index well" at Fairland, in northeastern Montgomery County, was six inches above the long-term average, with water standing at 10.6 feet below the land surface at the end of March.

Survey hydrologists said the March average daily flow of the Potomac River, source of most of the region's water supply, was 16.7 billion gallons a day, compared with a long-term average of 15.8 billion gallons. The range during the month was sharp, from about 8.1 billion gallons March 18 to 45 billion gallons on March 21, after a storm. The Washington area uses about 300 million gallons for its water supply on a typical March day.

The improved situation was a result of "warmer temperatures that allowed the precipitation and runoff to seep into the ground," survey hydrologist Myron Lys said.