Plus, according to the USGS, 1.9 million people are supplied by groundwater from the High Plains aquifer, with total public-supply withdrawals of 315 million gallons per day.

But intense use of the aquifer for agricultural irrigation during the last half century and global warming are threatening this once-abundant sea of underground water, according to some experts.

According to Don Wilhite of the National Drought Mitigation Center at the University of Nebraska-Lincoln, use of the aquifer began at the turn of the century. Since World War II, withdrawals from the aquifer steadily increased.

Two main factors are responsible. More crops were planted with the advent of manufactured nitrogen fertilizer after World War II, along with improved crop varieties and government crop-support programs that intensified the planting of major crops, such as corn, soybeans, wheat and cotton.

Also, more power engines could pump more water out of the aquifer as crop acres expanded.

"The withdrawal of this groundwater has now greatly surpassed the aquifer's rate of natural recharge," Wilhite said.

Another factor, according to Wilhite, is that much of the land beneath the High Plains aquifer is subject to climate extremes, such as the current drought impacting the Great Plains from Texas through Kansas and into the Dakotas.

Global warming, which many scientists around the world believe is occurring, is intensifying those extremes.

"The dependence on groundwater introduces an element of control by society over water resources and their use," he said.

Wilhite said the rates of drawdown and recharge of the aquifer varies from one locale to another. That's because the depth of the aquifer and its natural thickness varies.

"Some places overlying the aquifer have already exhausted their underground supply as a source of irrigation," he said. "Other parts have more favorable saturated-thickness and recharge rates, and so are less vulnerable."

For example, depletion of the aquifer in Texas and Kansas varies from Nebraska, which has had a more favorable recharge rate than other areas of the aquifer.

In Nebraska, some areas have allowed too many wells to be drilled. Those wells pump out more water than nature puts in the aquifer.

These factors have to be taken into account from region to region:

Yen Thickness of the bedrock between surface and the pool of underground water.

Yen Depth to groundwater.

Yen If more water is taken out of the aquifer than comes in, then recharge varies.

Yen If more water is being taken out of the ground than is coming in each year, the groundwater recharge is going to diminish each year.

In western Nebraska, like western Texas, the annual rainfall is minimal compared to eastern Texas and Nebraska. But, if the ground is more porous, more water enters the aquifer. That's why in the Sandhills of Nebraska where the vast pool of groundwater lies, the ground is more porous and there is more filtration of water. While the climate is very dry in the Sandhills, that aquifer took millions of years to gather the vast amount of water it contains. It was a dynamic process until man decided to mine it at ever increasing rates.

As climate change impacts the globe, areas that once had more favorable recharge rates have now implemented various controls on groundwater use from moratoriums to monitoring programs to groundwater management plans.

Wilhite said the depletion of the aquifer represents a change in the water balance of the Great Plains region, as would the suggested impacts of global warming.

"With a warming, soil moisture in the region would be reduced, as would rainfall," he said.

As the challenges mount for the aquifer, technology, conservation and good farming practices are helping to manage this resource better.

One way is to understand its complexities and delicate natural relationships.

That is a goal of a five-year investigation called the Platte River Cooperative Hydrology Study. This computer-modeling system was developed to help researchers understand the relationship between groundwater and surface water along the Platte River Basin.

While the High Plains aquifer covers a large part of Nebraska and has the bulk of the aquifer's groundwater reserves, the success of the study has implications for the entire eight-state area the aquifer covers.

The study came about as the result of the proposed Cooperative Agreement between Nebraska, Colorado, Wyoming and the U.S. Department of Interior.

One of the people who worked hard to develop the modeling system is Duane Woodward, Central Platte Natural Resource District hydrologist.

Woodward said the study gathered data to determine the impact groundwater pumping has on river and stream flows in the Platte River Basin.

According to a USGS report in 2000, irrigation use of groundwater amounted to 7.4 billion gallons per day or 94 percent of total groundwater use.

"We knew from the cooperative agreement program on the Platte River the state was working on with Colorado and Wyoming that we needed to be able to come up with what the depletion effects were from groundwater pumping," Woodward said.

He said study was set up to develop a database and models that act as tools to analyze groundwater and surface water interaction.

The database includes information on groundwater levels and surface flow data along the Platte River from more than 50 years of record-keeping, along with building a comprehensive understanding of the geology that impacts groundwater levels.

For example, if pumped out over the United States, the aquifer would cover all 50 states with 1.5 feet of water. Its 3.3 billion acre-feet of fresh water would fill Lake Huron, with enough water remaining to fill one-fifth of Lake Ontario.

The three-state cooperative agreement, which still hasn't been approved, is designed to ensure beneficial water flows in the Platte River for wildlife.

The result of over-development of the High Plains aquifer has caused some states, including Nebraska, to begin to place restrictions on the amount of groundwater pumped, the drilling of new wells, and the spacing of groundwater wells.

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A good example of the type of challenges facing aquifer use, while a microcosm of the vast High Plains aquifer area, is the Central Platte Natural Resources District, which covers about 2 million acres along the Platte River in Central Nebraska.

There are about 17,000 irrigation wells in the Central Platte NRD that irrigate nearly 1 million acres of crop land. In some counties in the NRD, such as Hall County, for more than 90 percent of the cropland, irrigation is its prime water source.

In 2003, the Central Platte NRD board of directors approved a temporary suspension of new well drilling in anticipation of the state law passed last year giving the state authority to determine whether an NRD is fully or "over appropriated" in its groundwater use.

"Fully appropriated" means that the amount of groundwater being pumped out of the ground equals the amount of recharge to the aquifer. "Over appropriated" means the amount of water being pumped out of the aquifer is more than what the recharge is.

Once Nebraska determines a district is to be fully appropriated, the NRD suspends the drilling of all new groundwater wells. While this has yet to happen, technically when a NRD is found to be over appropriated some existing groundwater wells could be shut down.

The state has determined that the Central Platte NRD, along the Platte River, is fully appropriated for groundwater pumping.

Woodward said the well-drilling moratorium is more out of concern about the use of water from the High Plains aquifer that would have flowed into the river system.

"It's the inner connection between groundwater and surface water that is the concern," Woodward said.

Many of the streams flowing through portions of the High Plains aquifer are fed by groundwater sources. But as more groundwater wells have been drilled, that has decreased stream flows that are also vital sources of water for surface irrigation, wildlife and other human water uses.

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Woodward said the study groundwater model is just getting to the point where it's beginning to estimate stream flow depletion due to groundwater use.

Global warming is also a concern as climate change is lessening snowfall along the Front Range of the Rocky Mountains. Melt-off of that snowpack is a huge source of recharge for the aquifer. Another source of recharge is precipitation in the Great Plains states that the aquifer lies below. Again, global climate change is increasing the intensity of drought and diminishing vital recharge of the aquifer.

"If global warming is occurring, then at some point whatever that does in reduction of rainfall there is a concern because our aquifer system is basically filled because of excess rainfall," Woodward said.

He said study will help officials better regulate water usage when models indicate a pending crisis. Overall, NRDs and farmers will be better able to manage groundwater and surface water resources.