Evaluation of SHAW Model in Simulating Energy Balance, Leaf Temperature, and Micrometeorological Variables within a Maize Canopy

Understanding and simulating plant canopy conditions can assist in better acknowledgment of plant microclimate characteristics, its effect on plant processes, and the influence of management and climate scenarios. The ability of the Simultaneous Heat and Water (SHAW) model to simulate the surface energy balance and profiles of leaf temperature and micrometeorological variables within a maize canopy and the underlying soil temperatures was tested using data collected during 1999 and 2003 at Yucheng, in the North China Plain. The SHAW model simulates the near-surface heat and water movement driven by input meteorological variables and observed plant characteristic (leaf area index [LAI], height, and rooting depth). For 1999, the model accurately simulated air temperature and relative humidity in the upper one-third of the canopy, but overpredicted midday temperature in the lower canopy. For 2003, although the surface energy balance was simulated quite well, radiometric canopy surface temperature and midday leaf temperature in the upper portion of the canopy were overpredicted, by approximately 5 C. Model efficiency (the fraction of variation in observed values explained by the model) for leaf temperature in the lower two-thirds of the canopy ranged from 0.82 to 0.90, but fell to 0.38 for the uppermost canopy layer. Weaknesses in the model were identified and potentially include: the use of K-theory to simulate turbulent transfer within the canopy; and simplifying assumptions with regard to long-wave radiation transfer within the canopy. Model modifications are planned to address these weaknesses. KNOWLEDGE of conditions near the soil–atmosphere interface is of key interest to many areas of research. The near-surface microclimate controls vital plant biological processes such as photosynthesis, respiration, transpiration, and crop damage from extreme temperatures. Canopy temperature reflects plant physiological conditions, not only by relating to air temperature, but also to stomatal opening, vapor diffusion resistance, and overall plant stress. Understanding processes of heat and water transfer within the plant canopy can assist in better acknowledgment of microclimate characteristics and their influence on plant processes. The ability to predict microclimatic conditions within the soil-plant-atmosphere system enhances our ability to predict plant response to microclimatic conditions and to evaluate management and climate scenarios (Gottschalck et al., 2001; Pachepsky and Acock, 2002; Yu et al., 2002, 2004). The surface energy balance describes the partitioning of net short and long wave radiation into latent, sensible, and soil heat fluxes which form the basis for simulating water and heat transfer and are the driving factors for C and N circulation. Transport of mass and energy between the land and atmosphere is an increasing area of interest as the need to better represent surface–atmosphere interactions in climate and atmospheric circulation models increases. Researchers have struggled with describing heat and mass transfer between the atmosphere and vegetated surfaces for more than 35 yr (Waggoner and Reifsnyder, 1968) and have developed several models ranging widely in complexity (Goudriann andWaggoner, 1972; Norman, 1979; Shuttleworth and Wallace, 1985; Kustus, 1990; Massman and Weil, 1999). Comprehensive models capable of simulating microclimate within the canopy typically employ one of two theories. Gradient (orK-theory) models (Norman, 1979; Flerchinger et al., 1998; Mihailović et al., 2002) define heat andmass fluxes within the canopy as the product of a concentration gradient and the eddy diffusivity, K. Considerable effort has been expended to estimate eddy diffusivities within the canopy (Ham and Heilman, 1991; Jacobs et al., 1992; Huntingford et al., 1995; Sauer et al., 1995; Sauer andNorman, 1995). TheKtheory has come under criticism for not predicting counter-gradient fluxes (Denmead and Bradley, 1985). Lagrangian trajectory theory (L-theory; Raupach, 1989) has been proposed as an alternate to K-theory, and recently several L-theorymodels have been developed (van den Hurk and McNaughton, 1995; Massman and Weil, 1999; Warland and Thurtell, 2000). Wilson et al. (2003) compared K-theory and L-theory approaches and concluded that both approaches performed equally in simulating surface energy components. The SHAW model, which is based on K-theory, was originally developed by Flerchinger and Saxton (1989b) and modified by Flerchinger and Pierson (1991) to include transpiring plants and a plant canopy. Its ability to simulate heat, water, and chemical movement through plant cover, snow, residue, and soil for predicting climate and management effects on soil freezing, snowmelt, soil temperature, soil water, evaporation, transpiration, energy flux, and surface temperature has been demonstrated (Flerchinger and Hanson, 1989a; Flerchinger and Pierson, 1991; Xu et al., 1991; Flerchinger et al., 1994, 1996a,b, 1998; Hayhoe, 1994; Flerchinger and Seyfried, 1997, Kennedy and Sharratt, 1998; Duffin, W. Xiao and Y. Zheng, Dep. of Environmental Sciences, Nanjing Univ. of Information Science & Technology, Nanjing 210044, China; Q. Yu, Institute of Geographical Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China; G.N. Flerchinger, USDA-ARS, Northwest Watershed Research Center, 800 Park Blvd., Suite 105, Boise, ID 83712. Received 2 May 2005. *Corresponding author ([email protected]). Published in Agron. J. 98:722–729 (2006).