An assessment of storage terms in the surface energy balance of maize and soybean
Introduction
Modeling of the soil–plant–atmosphere system has gone well beyond the original goal of daily predictions of water lost through evapotranspiration. Models of the plant biosphere are now used to assess carbon assimilation rates, evapotranspiration (Anderson et al., 2000), and the uptake of ozone, nitric acid vapor, and sulfur dioxide (Meyers et al., 1998). Many of these land-surface models (LSM) have been integrated into numerical weather-prediction models to provide more accurate predictions of the input of heat and water vapor into the lowest level of synoptic models. Over the last decade, major improvements have been made not only in the treatment of soil moisture and soil–water transfer (Boone et al., 2000, Noilhan and Planton, 1989), but also in the parameterizations of the canopy physiology. The incorporation of biochemistry into the models of stomatal conductance (Collatz et al., 1991) has placed the canopy response functions on a more scientific foundation, although factors such as the impact of leaf temperature and water stress are still empirically derived.
The current generation of LSMs have further developed the modeling of energy and mass exchange in the soil and plant canopy domains to provide an estimate of the net exchange of heat, water vapor and carbon dioxide between the surface and the atmosphere. However, compared to model predictions, the measured energy balance of many sites is often not closed (Wilson et al., 2002). Potential reasons for the lack of closure include loss of low frequency components of the flux, averaging procedures, and lack of an accurate accounting of all storage terms. Assessments of the canopy heat storage terms for forest ecosystems have been shown, at times, to be a significant contribution to the total surface energy balance (McCaughy and Saxton, 1988). Similar assessments for agricultural ecosystems are few (Mayocchi and Bristow, 1995), even though the problem of closure of the surface energy balance is still a concern (Wilson et al., 2002) for both forest and agricultural experimentalists.
In this paper, an assessment is made of the canopy heat and photosynthesis terms in the surface energy balance in fully grown corn (Zea maize L.) and soybean [Glycine max (L.) merr.] crops. These terms are usually neglected in models of agricultural ecosystems. This analysis is an attempt to account for not only the storage of heat in the soil, but also the storage of heat by the canopy biomass and water content, as well as the net energy flux consumed in the photosynthetic process.
Section snippets
Soil/canopy energy storage terms
Heat/energy storage terms of the soil–plant–canopy ecosystem can be examined from the complete steady state surface energy balance equation of the system defined aswhere the net radiation (Rn) per square meter land surface is balanced by the sum of the sensible (H) and latent (LE) heat fluxes to the air, and ground heat (G) flux, in addition to the energy fluxes for photosynthesis (Sp), canopy heat storage in biomass and water content (Sc), and ground heat storage above the
Evaluation of the energy storage terms
To assess the magnitude of the storage terms, data from the 1999 growing season were analyzed for an assessment of energy storage for maize. Data from year 2000 were used to assess the magnitude of energy storage for soybeans. The seasonal trend of plant water and total above ground biomass for maize reveals a sharp increase after day of year (DOY) 150 with water content reaching a maximum near 7 kg m−2 (Fig. 1). As the canopy senesced, water content dropped to approximately 4 kg m−2 by DOY 250.
Conclusions
Our understanding of the soil–plant–atmosphere continuum hinges on our ability to measure and account for all the critical processes that occur in this complex environment. The inability to account for all of the available energy in the surface energy balance has and continues to baffle researchers. Historically neglected as a significant component of the surface energy balance, canopy heat and photosynthetic storage, when combined with the soil heat component, is shown here to be a significant
Acknowledgements
The authors wish to acknowledge Richard Lawford and Jin Huang from NOAA’s Office of Global Programs/GAPP for supporting the commitment to obtain long term measurements in the GEWEX program. The views expressed in this paper are those of the authors and do not necessarily reflect those of the funding agency or the Illinois State Water Survey.
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