MISSISSIPPI STATE -- Mississippians are not in the habit of keeping track of water supplies, but a second summer of drought is taking its toll on the underground water stores.

Most of the state gets its water from underground aquifers. A few areas use surface water for their supply, but most municipalities dig wells to serve the needs of communities, industries and agriculture.

Jim Thomas, agricultural engineer with Mississippi State University's Extension Service, said aquifers store water in sand and gravel layers confined between layers of clay or solid rock. These geologic formations overlap each other at varying depths and extend for miles.

"Aquifers store water in the soil, sand and gravel in sufficient volume that when you drill a well, you can get enough water to sustain an adequate supply for industrial, agricultural or domestic use," Thomas said.

These aquifers typically recharge themselves from rain, rivers and watersheds in their area. But drought since the start of last summer is affecting their ability to refill.

"We're not getting much recharge at this time. We're withdrawing more than is being put in," Thomas said. "It's not something that has occurred this summer, but is a carryover from a year and a half without adequate rain."

In Mississippi, the only aquifer that historically has had trouble keeping up with demand is the one that supplied Tupelo. Thomas said when this started years ago, the city began to draw water from the Tombigbee River and dedicated the aquifer to smaller communities in the area.

Ground and surface water each have their advantages. Except in a drought, surface water is replaced quickly by rainfall, while ground water reservoirs take longer to recharge. Ground water is stored in tremendous quantities, is refilled by sources in addition to rain, and doesn't suffer from evaporation that surface waters face. Ground water also often requires less treatment before it can be used.

While the water supply in Mississippi is not at risk today, Thomas said some farmers tapping the alluvial aquifer under the Delta have had to set their pumps a little deeper to reach the water they need.

"We're blessed with abundant ground water supplies in this state," Thomas said. "At some point, we'll probably need to develop more surface water supplies."