Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 8, 1998
Publication Date: N/A
Interpretive Summary: Previous studies of plant growth and water use, most using enclosed chambers, have found evidence that the increasing atmospheric CO2 concentration may change the amount of water used by plants. Such a change in plant water use could impact regional water supplies and require farm managers to modify their management practices. In order to determine the magnitude of such changes under field conditions, plots of wheat were exposed to elevated CO2 concentrations using free-air CO2 enrichment (FACE) apparatus. Devoid of walls, the FACE approach is the most natural technique available to conduct such research. Data were collected for four growing seasons at ample water and fertilizer and for two seasons when soil nitrogen was limited. The FACE treatment increased daytime foliage temperatures about 0.6 and 1.1 C (1.1 and 2.0 F) at high and low nitrogen, respectively, suggesting that optimal regions for wheat production could shift in the future because of the elevated CO2 alone, regardless of any climate change. Daily water use was consistently lower in the FACE plots, by about 6.7 to 19.5% for high and low nitrogen, respectively. These results suggest that future water use requirements will decrease slightly, provided that future changes in climate are not adverse. This information, therefore, will be useful for farmerrs in mkaing better manaagement decisions for crop production while conserving water.
In order to determine the likely effects of the increasing atmospheric CO2 concentration on future evapotranspiration, ET, plots of field-grown wheat were exposed to concentrations of 550 ppm CO2 (or 200 ppm above current ambient levels of about 370 ppm) using a free-air CO2 enrichment (FACE) facility. Data were collected for four growing seasons at ample water and fertilizer (High-N) and for two seasons when soil nitrogen was limited (Low-N). Measurements were made of net radiation, Rn; soil heat flux; air and soil temperatures; canopy temperature, Ts; and wind speed. Sensible heat flux was calculated from the wind and temperature measurements. ET, i.e., latent heat flux, was determined as a residual in the energy balance. The FACE treatment increased daytime Ts about 0.6 and 1.1 C at High- and Low-N, respectively. Daily total Rn was reduced by 1.3% at both levels of N. Daily ET was consistently lower in the FACE plots, by about 6.7 and 19.5% for High- and Low-N, respectivley.