Submitted to: Book Chapter
Publication Type: Book / chapter
Publication Acceptance Date: 11/16/2007
Publication Date: 12/31/2008
Citation: Yu, Q. and G.N. Flerchinger. 2008. Extending Simultaneous Heat and Water (SHAW) Model to Simulate Carbon Dioxide and Water Fluxes over Wheat Canopy. Chapter 7 In: In L.R Ahuja, V.R. Reddy, S.A. Saseendran, and Q. Yu (eds.) Response of crops to limited water: Understanding and modeling water stress effects on plant growth processes. Advances in Agricultural Systems Modeling Ser. 1. ASA, CSSA, SSSA, Madison, WI. 436 pp. Interpretive Summary: Increased levels of carbon dioxide in the atmosphere can influence plant response and water usage. Better understanding of these interactions can lead to better understanding of plant ecological processes. The Simultaneous Heat and Water (SHAW) Model is computer model of heat and water exchange at the surface, but has no provisions for exchange and use of carbon dioxide by plants. The objective of this study was to link the SHAW model with a photosynthesis model to capture the interaction between carbon dioxide on water use by plants. In doing so, the SHAW model was extended to simulate carbon dioxide exchange and will be more comprehensive in simulating plant response for to environmental changes, such as increase in atmospheric carbon dioxide and climate warming.
Technical Abstract: Energy, water and CO2 flux at the soil-atmosphere interface is a key interest among ecosystem researchers. The Simultaneous Heat and Water (SHAW) Model describes radiation energy balance, heat transfer and water movement within the Soil-Plant-Atmosphere Continuum, but has no provisions for carbon assimilation. This study coupled the components of solar radiation and water transfer within a plant canopy in SHAW with a biochemical photosynthesis model. The SHAW model provides leaf water potential to the photosynthesis model to calculate intercellular CO2 (Ci) through stomatal control in each layer within the canopy, and then provides solar radiation, air temperature and humidity used to calculate photosynthetic rate (Pn) within each canopy layer. Stomatal conductance (gs) was calculated by a revised Ball-Berry model, describing the relationship between gs and Pn, which was a feedback from the photosynthesis model to SHAW to calculate energy and water transfer, and in turn the leaf water potential. After including the relationship between stomatal conductance and photosynthetic rate, computed stomatal conductance was able to decrease in response to CO2 increase. The photosynthesis model was validated, and photosynthesis, transpiration, stomatal conductance, and Ci at a leaf show appropriate response to changes in light and CO2. The extended SHAW (SHAW-Pn) model was capable of simulating CO2 flux over plant canopies. It performs excellently in simulating net radiation, sensible and latent heat, and CO2 fluxes over winter wheat field in the North China Plain (36°57’N, 116°36’E, 28 m above sea level). The root mean square error (RMSE) of simulation for net radiation, latent and sensible heat fluxes is 38.4, 46.0, and 26.3 W m-2, respectively. The RMSE of CO2 flux simulation is 0.16 mg m-1 s-1. This model describes biophysical, and biochemical processes and physiological regulation of water and carbon cycles in the ecosystem, which can be a framework of vegetation response to interaction between climate warming and atmospheric CO2 increase if a plant growth module is included.