Paleoclimatic studies of tree rings and archaeological remains suggest the US heartland has seen numerous severe droughts over the last 2000 years, the commerce department's National Oceanic and Atmospheric Administration (NOAA) study said.

"Conditions that could lead to severe droughts, such as that of the late 16th century, could recur in the future, leading to a natural disaster of a dimension unprecedented in the 20th century," said Jonathan Overpeck, head of NOAA's paleoclimatology programme.

Scientists want state planners to take into account a possible upswing in unpredicted droughts and to better conserve well water for irrigation while diversifying farm economies.

By 2020, quality water from the High Plains Ogallala Aquifer supplying 30 per cent of the ground water used for US irrigationwill be drained, they warn.

University of Colorado scientist Connie Woodhouse, who works at NOAA's National Geophysical Data Centre in Boulder, Colorado, said more droughts are to be expected.

"Paleoclimatic records of the past 400 years strongly indicate that the severe droughts of the 20th century, the 1930s `Dust Bowl' and the 1950s drought, were not unusual events," she said.

"(They) suggest that we can expect to have droughts of this magnitude once or twice a century."

Drought in the Plains states, which stretch North from Texas to the Dakotas, has had a special resonance in American culture since the 1930s event, which scientists say lasted eight years.

Author John Steinbeck's "The Grapes of Wrath" immortalised the plight of the farmer Okies, or Oklahomans, who suffered from the agricultural devastation which sent thousands fleeing to California or elsewhere in search of a better life.

The era left deep scars for those who can recall the migrations of destitute farmers from their land tosurvive, but they may not be the last.

Woodhouse said new evidence shows at least two protracted, multi-decade droughts hit portions of what is now the South and West United States in the 13th and 16th centuries.

Probability and signs of modern global warming led the scientists to warn that long droughts could return to the US.

Despite debate over the human contribution to the global warming phenomenon, Overpeck said warming and modern land use practices still meant the Great Plains were nevertheless more susceptible to severe drought.

Droughts continue to elude forecasters. A 1998 drought in Texas and Oklahoma surprised scientists, who foresaw wetter than normal conditions for the region.