Water in the atmosphere eventually falls as precipitation on the earth's surface and starts the cycle all over again. Like water in streams and rivers, ground water moves, but at a very slow rate. Ground water flows from areas where precipitation percolates down to the water table, called recharge areas, to locations where it flows out of an aquifer and becomes surface water. Most ground water, however, flows directly into streams, rivers, lakes, and wetlands through the stream bed or the bottom of the lake or wetland. Most of the flow in streams and rivers during drought times is ground water discharging from aquifers into the stream channel.
Base flow can also be a significant component of stream flow during normal times.
In many streams, base flow sustains aquatic life during prolonged dry spells. Over 98 percent of the available fresh water on earth is ground water. This includes large municipalities, such as Dayton, Canton, and Columbus, which rely on ground water to provide all or part of their water supply, and over 700,000 domestic water wells supplying individual homes throughout the state. Total ground water pumped for all uses in Ohio is 730 million gallons per day. That's over 266 billion gallons per year, enough water to flood the entire City of Columbus to a depth of almost 7 feet.