Four central U.S. states hardest hit by this year's extraordinary drought saw conditions worsen over the last week, as record-breaking heat and dry weather continued to wither crops, toast pastures and dry up streams for the second month.

The U.S. Drought Monitor released this morning showed dry conditions intensifying and expanding across Nebraska, Kansas, Oklahoma and Missouri, even as they receded slightly elsewhere.

More than half the United States, 52 percent, remains in a state of moderate drought or worse.

Exceptional drought conditions expanded over the last week in Kansas, Missouri, Nebraska and Oklahoma, even as lesser drought conditions receded elsewhere. Click on map to enlarge. Map courtesy of the University of Nebraska, Lincoln.

Already by early last month, the extent of this dry spell was greater than any other measured in the 12-year history of the Drought Monitor, drawing comparisons to the historic drought of 1988.

"Most people are saying this is the worst in decades," said Mark Nelson, director of commodities for the Nebraska Farm Bureau.

In the span of one week ending Tuesday, the total land area suffering from exceptional drought conditions -- the worst possible -- expanded from 39 percent to 63 percent of Kansas, from 14 percent to 36 percent of Missouri, from 16 percent to 39 percent of Oklahoma and 3 percent to 23 percent of Nebraska.

South Texas also saw a patch of extreme drought expand, while extreme and exceptional drought grew in eastern Colorado.

Three of the four states seeing worsening drought sit atop the Ogallala Aquifer, a vast store of groundwater beneath the Great Plains that doesn't refill easily and has long been threatened by overpumping. Concerns are rising that as farmers pump more water to salvage crops, the aquifer could be dealt a significant blow this year.

"There's very little recharge coming to the Ogallala anyway in many places, so, yes, it will increase the concerns in some areas," said William Cunningham, assistant chief of the Office of Groundwater at the U.S. Geological Survey.

Today's report, however, did offer some hope for surrounding regions.

Improvements are expected over the coming months in the Southwest, the Corn Belt, the upper Mississippi Valley, the lower Great Lakes, the Ohio Valley and the Southeast, according to Brent Fuchs, a climatologist at the National Drought Mitigation Center.

"Outside of those areas, drought will persist over much of the central and southern Plains and into the western United States," Fuchs said in a statement.

Effects of the drought are being felt across farms and waterways.

For now, at least, water levels in the Mississippi River are causing barges to run aground more frequently. This forces ships to take on smaller loads, choking commercial waterways, according to NBC News.

Corn crops are being wiped out, harvested early or, in some cases, abandoned altogether, as farmers move to file crop insurance claims to cover their losses, according to farm groups. The Department of Agriculture recently lowered crop yield forecasts due to drought.

"Yesterday, we heard reports from guys that are getting 50 to 70 bushels per acre when they normally would get 120 to 130," said Jay Rempe, vice president for government relations at the Nebraska Farm Bureau.

Cattle ranchers were suffering even worse because of dwindling water supplies, according to Nelson of the Kansas Farm Bureau.

"We're seeing such little streamflow in some of our creeks and rivers that some of our ponds are becoming stagnant, which increases the risk of blue-green algae," Nelson said. "That's poisonous to livestock. We've actually had some cattle deaths from that."