One winter's worth of rainfall doesn't always replenish water table

One winter's worth of ample rainfall doesn't necessarily make for a replenished water table, says Jim Hairston, Alabama Cooperative Extension System water quality scientist.

Much of the rain that fell across Alabama this winter ended up being washed into lakes, rivers and streams instead of seeping into the soil and eventually reaching underground water reservoir or aquifers, says Hairston.

“People see these streams and reservoirs filling up after significant rainfall over several months, and they think our water tables are recharged and ready to go. But that isn't the case,” he says.

“What they don't know is that a lot of this water, especially during heavy rainfall, ran off the ground surface and into rivers and streams faster than it could be absorbed into soil and carried further down to underground water supplies,” he adds.

Coupled with this problem, says Hairston, is the stress on aquifers stemming from private and municipal water use from wells and agricultural irrigation drawn from deep wells. In some cases, water is pumped out of the aquifers faster than it can be replenished from rainfall — a factor complicated by last summer's severe drought.

“People often think there is a never-ending supply of groundwater,” says Hairston. “However, if you're drilling more wells to provide this water to municipal drinking water and irrigation systems, you've got to think about the long-term effects, especially if it appears the groundwater isn't being recharged fast enough to replace what's being taken out.”

In some cases, he explains, the distance from the water recharge area (the point at which water is absorbed into and pumped from the ground) and the aquifer can be as far as 50 to 100 miles. Since water may flow only a few inches an hour, it make take months or even years, in some cases, before it reaches its final destination.

As a result, there often is a significant time delay from the point at which rainfall soaks into the soil to the point from which it is pumped from an aquifer.

Groundwater depletion is a far bigger concern in south Alabama, where greater use is made of groundwater than in north Alabama, which still depends primarily on surface water for most of its industrial and municipal drinking water needs, says Hairston.

The good news, he says, is that with the exception of Louisiana and Florida, Alabama normally receives more annual rainfall than any other state — 55 inches. Also, compared with other states, Alabama enjoys substantial groundwater supplies, despite the stress caused from last summer's drought.

As long as town's and cities continue enacting measures to conserve these resources, Hairston believes there will be plenty of groundwater available for future needs. A major focus of these measures probably will involve limiting commercial and industrial development in areas where primary aquifer recharge occurs.

“Whenever you remove natural vegetation from recharge areas and replace it with asphalt and concrete, you're potentially limiting the amount of rainwater that otherwise would soak into the aquifer,” he says.

If groundwater isn't a big enough concern, expanding population growth coupled with severe drought periods in some regions is placing an ever bigger strain or surface water supplies throughout most of Alabama.

Last year in Birmingham, for example, the almost complete withdrawal of water from Lake Purdy and Inland Lake, due to the extended summer drought, left municipal planners scrambling to find alternative sources of drinking water.

One solution would have involved building a pipeline to the Coosa River more than 30 miles away. However, this proposal enraged Coosa-area residents because water diverted from the river would have been emptied into the Cahaba River after waste treatment and never returned to the Coosa River.

The decision to divert water permanently out of the Coosa River into another river would have amounted to what is known as an “interbasin transfer,” which has become one of the most hotly debated water management issues in the world, says Hairston.

More and more city planners, he says, will be forced to adopt stringent conservation provisions to safeguard existing surface water and groundwater resources.

“One effective solution would be to build off-stream reservoirs near major rivers to trap water during high-flow periods. Once used, the water then can be treated and returned to the stream for future use.”

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