CHANGES IN MASS AND ENERGY TRANSFER BETWEEN THE CANOPY AND THE ATMOSPHERE: MODEL DEVELOPMENT AND TESTING WITH A FREE-AIR CO2 ENRICHMENT (FACE) EXPERIMENT.

Authors: MANUNTA, PAOLO - UNIV OF ALBERTA; GRANT, ROBERT - UNIV OF ALBERTA; FENG, YONGSHEN - UNIV OF ALBERTA; Kimball, Bruce; Pinter Jr, Paul; LAMORTE, ROBERT - US WATER CONS LAB; Hunsaker, Douglas - Doug; Wall, Gerard - Gary

Submitted to: Journal of Biometeorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/16/2001
Publication Date: 11/16/2001
Citation: N/A

Interpretive Summary: The CO2 concentration in the atmosphere is increasing and expected to double near the end of this century. The elevated levels of CO2 affect plant photosynthesis and also cause a partial closure of the stomata in plant leaves through which the plant exchanges CO2 and water vapor with the atmosphere. In order to predict what effect the elevated CO2 will have on future crop production and to aid in developing improved management strategies, crop growth simulation models are being developed. This paper reports an approach to modeling stomatal controls on mass (water vapor and CO2) and energy transfer between crop canopy and the atmosphere. A successful test of the model was conducted utilizing results of an experiment where open-field-grown wheat was exposed to elevated levels of CO2 using free-air CO2-enrichment (FACE) technology at ample and limited levels of soil water. Both measured and simulated values of water use were reduced by about 6% at ample water and by about 2% when water was limited due to elevated CO2 concentrations of 550 ppm (such as expected near the middle of this century). This work will benefit both future growers and consumers of wheat and wheat products.

Technical Abstract: The rationale for this study is found in the probably higher temperatures and changes in rainfall patterns that are expected in the future as a result of increasing levels of CO2 in the atmosphere. In particular, higher air temperatures may cause an increase in evapotranspiration demand while a reduction in rainfall could increase the severity and duration of drought in arid and semi-arid regions. Representation of the water transfer scheme includes water uptake by roots and the interaction between evapotranspiration and CO2 enrichment. The predicted response of a spring wheat (Triticum aestivum L. cv. Yecora rojo) canopy in terms of energy exchange processes to elevated atmospheric CO2 level was tested against measurements collected at the FACE (Free Air Enrichment Experiment) site in 1994. Simulated and measured canopy conductances were reduced by about 30% under elevated [CO2] caused reductions in both simulated and measures seasonal water use of 6% under optimum and 2% under suboptimum irrigation. The soil-plant-atmosphere water transfer scheme proposed here offers several advances in the simulation of land surface interactions. First, the stomatal resistance model minimizes assumptions in existing land surface schemes about the effects of interactions among environmental conditions (radiation, temperature, CO2) upon stomatal behavior. These interactions are resolved in the calculation of CO2 in which processes are already well understood.