VALENTINE, Neb. - Driving south this summer through north-central Nebraska's lonely Sand Hills cattle country, motorists will notice a soft, irregular tapping on the windshield and under the floorboards.
It sounds like the first, heavy drops of a summer thundershower. But it's not raining. Nebraska is one of 14 Western states being seared by extreme drought this year.
A more careful look reveals grasshoppers by the thousands, crossing the sun-blistered asphalt and leaping into the path of approaching cars. The legions of hungry bugs are one more sign of the drought that has turned these rolling grasslands from soft green to a brittle tan.
The deepening drought worries the scattered ranchers who depend on the rangeland to feed and fatten cattle into October.
It also has Nebraska's geologists talking. The Sand Hills region is the hemisphere's largest grass-stabilized dune field - 12 million acres of rolling, Sahara-like sand dunes, held in place by a fragile carpet of prairie grass.
Scientists who have been sinking coring tools deep into the old dunes have found growing evidence that prolonged droughts as recent as 900 years ago have killed the upland grasses and exposed the dunes to the incessant prairie wind, setting them free.
What they don't know is how severe or how prolonged the loss of moisture must be before the dunes start moving again. "Let's hope that the present drought we're in is just a blip and not the beginning of dune time," said Jim Swinehart, a research geologist with the University of Nebraska.
At the least, it is a drought unlike any that Gerald and Marianne Beel have seen before.
The Beels have worked the Duck Bar Ranch, southwest of Valentine, for 54 years. Five miles from their nearest neighbors, they revel in the quiet and in the subtle beauties of High Plains wildflowers and sunrises.
The lack of rain this year has parched the rangeland and diminished the yield of many Sand Hills hayfields by a half to two-thirds, says the state cooperative extension service.
With normal rainfall, the Beels can usually put up 3,000 bales of hay to get their beef cattle through the winter. "I don't think we'll get 1,000 this year," said Marianne Beel, 74.
Good hay is so scarce this summer that the Beels would have to pay $140 to $200 a ton for it - three to five times the price two years ago. "This is bad; this is really bad," Marianne Beel said. "All of us are worrying."
Bud Stoltzenberg, University of Nebraska extension educator for Cherry County, said some ranchers might wean their calves early to reduce cows' nutritional needs. Or they might sell off parts of herds, or look for cheaper, alternative feeds.
Beel and her husband are arranging to truck their 500 cattle 130 miles south to a place on the North Platte River this fall. There, they can forage on the cornstalks left in irrigated or drought-damaged fields. It isn't cheap to truck 500 head that far. But short of selling the herd, "cornstalks is about the best alternative," Beel says.
This is always an arid region, averaging 18 to 20 inches of rain a year. It's mostly too dry and sandy for food crops, and it takes 20 acres of prairie grass to support a cow-calf pair. That's why ranches are measured by the tens of thousands of acres and why Cherry County has just 6,145 people on a landscape bigger than Connecticut.
But this year, parts of the area have seen just 7 inches of rain. Some ranches have reported less than an inch. Add to that 100-degree-plus temperatures and high winds, and "what moisture there is, is just evaporated out," Beel said.
For as long as anyone here can remember, the grass that covers the Sand Hills has been watered largely by moisture carried by weather systems from the Gulf of Mexico. It is weeded by fire and trimmed by grazing animals. The migrating herds of elk and bison have been replaced by fenced cattle. But the velvety hills seem permanent.
However, geologists have long recognized that the grass cover has not always been there. It is merely anchoring a vast, undulating field of windblown sand nearly twice the size of Massachusetts.
"This is the great American desert," says University of Nebraska geologist Jim Goeke.
The dunes' origins aren't fully understood. Some geologists believe they formed more than 12,000 years ago, after the end of the last Ice Age, as sand blown from the shallow bed of the Niobrara River in northern Nebraska piled up to the south and east. From the air, the Sand Hills provide vistas of crescent-shaped or rolling hills - some as high as 400 feet and all covered with grass. Some formed into ranks of long narrow windrows stretching as far as 20 miles.
Surprisingly, this huge sand desert holds vast stores of ground water.
Little of the rain and snowmelt here runs off the land. It filters quickly through the sand and feeds one of the richest aquifers in the world - the Ogallala. The resulting high water table is visible in the numerous lakes, marshes and bubbling springs that green the inter-dune valleys. So, even as the upland grasses wilt, ducks, geese and heron abound in the lowland cattails. Fed by the Ogallala, the Loup, Dismal and other Sand Hills rivers are among the most constant in the world.
What scientists didn't know was how long this pattern had prevailed. How recently were these dunes active? Could they be again?
To find out, Goeke, Swinehart and their colleagues began driving coring tools deep into the Sand Hills. Examining those cores, "you're just turning up the pages of Earth's history," Goeke said. They revealed alternating layers of peat and eolian - windblown - sand.
Carbon dating of the peat and a new technique for dating the burial of sand grains called "optically stimulated luminescence" have shown that the dunes date back at least 12,000 years.
"Good times" marked by peat, pollen and seeds were interrupted repeatedly by dry periods and reactivation of the dunes. The "bad times" persisted between 8,000 and 5,000 years ago, returned between 4,000 and 2,000 years ago, and came back again 900 years ago.
Even the intervals of high dune activity did not drive all life from the Sand Hills. Swinehart and others have found bison footprints in the buried dune surfaces and evidence that some interdune valleys remained moist and green. But an upland cattle economy would not have survived.
Although scientists don't know when the next "dune time" will be, Swinehart said, "We think this is going to be the story again and again. The overall big thing that's controlling it is the climate."
The droughts of the 1930s weren't enough to set the dunes loose in the wind, he said. But "20 or 30 years of rainfall below 10 inches a year, and the dunes are bare."
"Given this past year's drought," he said, "I wonder what that sand dune is thinking."