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Dry summers are lowering Delta groundwater supplies

Agriculture.com Staff 12/04/2007 @ 8:03am

When rains don't meet the needs of crops in Mississippi's agriculturally rich Delta, producers rely on irrigation to meet their plants' needs, drawing water from the deep and plentiful alluvial aquifer below their soils.

An aquifer is a ground formation of coarse gravel and small rocks filled with water in the cracks and empty spaces. Aquifers are recharged slowly by underground water supplies fed by rainfall often hundreds of miles away.

In the Delta, the alluvial aquifer is only 30 to 40 feet below the ground surface. Tim Walker, a rice agronomist with the Mississippi Agricultural and Forestry Experiment Station (MAFES), says producers drill wells about 125 feet into the earth to tap into this abundant water supply and make up any shortage between what comes down as rain and what their crops demand.

"None of our producers have ever had a well run dry, but a lot of growers have commented that the wells are not as productive as before," Walker says.

Dean Pennington, executive director of the Yazoo Mississippi Delta Joint Water Management District, says despite its size and volume of water, recent droughts have lowered the level of the alluvial aquifer.

"History shows that long-term, we are taking more water out of the aquifer than is naturally recharged," Pennington said. "The summers of 2006 and 2007 were both dry, and we saw some fairly significant declines, from nine to 12 inches."

In the early 1980s, the U.S. Geological Survey established a network of 600 irrigation wells across the Delta. These are measured every April and October. Pennington said this data shows the impact irrigation has on Mississippi's groundwater levels.

"Our total water use from the aquifer is about 1.5 million to 2 million acre-feet of water per year," Pennington says. An acre-foot of water is the amount of water needed to cover an acre with one foot of water, or about 326,000 gallons. "We have a trend that has to be reversed."

Pennington adds dry years show up in historical data as lower groundwater levels.

"The deficit is about 300,000 acre-feet of water a year," he says. "Compared to some places, that is exceptionally good news. A 20% reduction in our groundwater use would balance our water budget."

Pennington said the major goals of the water management district are to promote water conservation and to develop new water supplies.

"Surface water supplies are much more quickly recharged than groundwater supplies," Pennington says. "We can't help store more water in the ground. Mother Nature has to be left alone on that. But by engineering, we can store more surface water."

An example of developing new water supplies is the effort to transfer water from the plentiful Tallahatchie River to the Quiver River, which runs through an area between the Sunflower and Yazoo rivers.

"It flows right over the area in the Delta where we have the most significant groundwater declines," Pennington says.

Nathan Buehring, rice specialist with the Mississippi State University Extension Service, says rice requires 30 or more inches of water a year. This makes rice the crop with the highest water demand of Mississippi's row crops.

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