One of the earliest climate-model studies to address this subject as it applies to the heartland of the United States was that of Manabe and Wetherald (1987), whose doubled-CO2 calculations yielded increased summer evaporation over the U.S. Great Plains that greatly reduced contemporary soil moisture contents there.  The calculations of Rind et al. (1990) similarly suggested severe drought conditions fully 50% of the time over the entire United States by the 2050s, while Rosenzweig and Hillel's (1993) simulations conjured up mid-21st century droughts more severe than the drought that produced the U.S. Dust Bowl of the 1930s.  Also concentrating on the mid-21st century, Manabe et al. (2004) find large percentage reductions in summer soil moisture from middle to high latitudes throughout most of the Northern Hemisphere, with reductions throughout most of the year in the southwestern United States.

In light of the fact that Mann and Jones (2003) claim that the warming of the globe over the 20th century was unprecedented over the past nearly 2000 years, one might logically expect there to already be evidence for the initial stages of the predicted drying of the U.S. heartland if the climate model predictions are correct.  Hence, in an attempt to seek such evidence in what could be considered an initial test of the climate model predictions, Mauget (2004) decided to look for what he calls "initial clues" that might be apparent in the observational streamflow record of the region.

The data for the study were obtained from the archives of the U.S. Geological Survey's Hydro-Climatic Data Network (HCDN) and pertain to 42 stations that cover the central third of the United States, ranging from the Canadian border on the north to the Gulf of Mexico on the south, with the most dense coverage being found within the U.S. Corn Belt.  Each station's record covered a minimum of 60 years, and all records ended in 1998.

So what did Mauget learn?  He reports finding "an overall pattern of low flow periods before 1972, and high flow periods occurring over time windows beginning after 1969."  Of the 42 stations' high flow periods, for example, he says that "34 occur during 1969-1998, with 25 of those periods ending in either 1997 or 1998," and that "of those 25 stations 21 are situated in the key agricultural region known as the Corn Belt."  He also reports that "among most of the stations in the western portions of the Corn Belt during the 1980s and 1990s there is an unprecedented tendency toward extended periods of daily high flow conditions, which lead to marked increases in the mean annual frequency of hydrological surplus conditions relative to previous years."  What is more, he notes that "in 15 of the 18 Corn Belt gage stations considered here at daily resolution, a more than 50% reduction in the mean annual incidence of hydrological drought conditions is evident during those periods."  Last of all, Mauget reports that "the gage station associated with the largest watershed area - the Mississippi at Vicksburg - shows more than a doubling of the mean annual frequency of hydrological surplus days during its 1973-1998 high flow period relative to previous years, and more than a 50% reduction in the mean annual incidence of hydrological drought condition."

In summarizing his findings, Mauget states that the overall pattern of climate variation he discovered "is that of a reduced tendency to hydrological drought and an increased incidence of hydrological surplus over the Corn Belt and most of the Mississippi River basin during the closing decades of the 20th century," noting further that "some of the most striking evidence of a transition to wetter conditions in the streamflow analyses is found among streams and rivers situated along the Corn Belt's climatologically drier western edge."

Do these findings represent the early stages of real-world climate change?  Mauget states that the streamflow results do indeed "suggest a fundamental climate shift, as the most significant incidence of high ranked annual flow was found over relatively long time scales at the end of the data record."  And that shift, as we hardly need to emphasize, is away from the droughty conditions long predicted to result from CO2-induced global warming in this important agricultural region of the United States.

References
Manabe, S., Milly, P.C.D. and Wetherald, R.  2004.  Simulated long-term changes in river discharge and soil moisture due to global warming.  Hydrological Sciences Journal 49: 625-642.

Manabe, S. and Wetherald, R.T.  1987.  Large-scale changes of soil wetness induced by an increase in atmospheric carbon dioxide.  Journal of the Atmospheric Sciences 44: 1211-1235.

Mann, M.E. and Jones, P.D.  2003.  Global surface temperatures over the past two millennia.  Geophysical Research Letters 30: 10.1029/2003GL017814.

Mauget, S.A.  2004.  Low frequency streamflow regimes over the central United States: 1939-1998.  Climatic Change 63: 121-144.

Rind, D., Goldberg, R., Hansen, J., Rosenzweig, C. and Ruedy, R.  1990.  Potential evapotranspiration and the likelihood of future drought.  Journal of Geophysical Research 95: 9983-10004.

Rosenzweig, C. and Hillel, D.  1993.  The Dust Bowl of the 1930's: Analog of greenhouse effect in the Great Plains?  Journal of Environmental Quality 22: 9-22.