Global Warming -- Real Or Perceived -- Could Strain Water Resources

The potential for varying rainfall could lead to a perceived threat among farmers, who would be tempted to use the irrigation equipment that they installed to recoup their investment when conditions only marginally dictate a benefit, says J. Wayland Eheart, a professor of environmental systems.

Such over-irrigation could reduce stream flow, critically affecting water quality and aquatic ecosystems such as fish populations, adds Edwin E. Herricks, a professor of environmental biology.

Eheart and Herricks are co-investigators of a three-year study funded by the U.S. Environmental Protection Agency to explore the vulnerability of Midwest water resources and aquatic ecosystems. They spoke during a symposium May 19 at the U. of I. Eheart also will chair a June 9 conference on "Water Management Programs for Humid Regions" sponsored by the American Society of Civil Engineers at the Hyatt Regency Hotel in Chicago.

The United Nations' Intergovernmental Panel on Climate Change in 1995 predicted an increase of global mean surface temperature of 1 to 3.5 degrees Celsius by 2100 and increased variances of precipitation in North America because of carbon dioxide emissions. Eheart and Herricks are using historical records of the Sangamon River watershed in Illinois and creating models of potential scenarios.

"The increase in variability of precipitation alone, without a change in mean precipitation, may increase irrigation and, in turn, decrease stream flows," Eheart said. "There is also the Chicken Little effect -- by which even perceived changes could lead to the same result."

Irrigation decisions are assumed in the model to be based on a "root-zone deficit trigger" -- the level of moisture deficit in top soil. Eheart's model adjusts for a variety of conditions and seeks an optimal trigger for using irrigation to assure desirable yields or to maximize profit.

Based on current corn prices, for example, irrigation likely would be used if annual precipitation averages decreased by 15 percent, Eheart said, resulting in the use of more than 4 inches of water -- an amount that is 50 percent of the mean annual groundwater recharge in some parts of the Midwest. In Illinois, as in some other Midwest states, farmers may withdraw water as they please from streams, although they can not install permanent pumping structures.

Over-irrigation could drive stream flows below the traditional minimum dilution flow, at which point waste being discharged is not diluted sufficiently to sustain water-quality standards. Under their model, Eheart said, "we could see up to 60 days per year of violating this standard, depending on the profit- or yield-maximizing strategies farmers choose to use."