Increasing Atmospheric Carbon Dioxide and Water Use Efficiency.

The Evapotranspiration (ET) Laboratory at Kansas State University was the first to carry out experiments with sorghum and winter wheat under elevated levels of carbon dioxide (CO₂) in the field. For three years (1984-1987), we grew sorghum or winter wheat under elevated levels of CO₂ in closed top chambers at a rhizotron facility near Manhattan, KS. When we started the experiments, the concentration of CO₂ in the atmosphere was 330 ppm. The four atmospheric CO₂ concentrations that we used were 330 ppm (control), 485 ppm, 660 ppm, and 795 ppm (for sorghum) or 825 ppm (for wheat). The CO₂ concentration in the air in 2007, the last year for which data are published, was 383 ppm. Because the concentration of CO₂ in the atmosphere has increased 53 ppm since we started our experiments, it is time to revisit the earlier data--in particular the data that dealt with water use efficiency (WUE)--to determine how much WUE has increased as a result of increased levels of CO₂. In general, the greatest reduction in water requirement (reciprocal of WUE) occurred between the control (330 ppm CO₂) and the first increment of CO₂ added to the air (485 ppm CO₂). That increment is 155 ppm CO₂. We already are one-third of the way to reaching that first increment. I have estimated what the water requirement of sorghum and wheat now is, based on the data collected for sorghum and wheat grown 1984-1987. Sorghum and wheat are now using 43 mL and 41 mL, respectively, less water to produce a gram of grain than they were in the 1984-1987 period. Increases in atmospheric CO₂ apparently have allowed production of more sorghum and wheat grain for the same amount of water applied.