Predicting canopy temperatures and infrared heater energy requirements for warming field plots

Authors: KIMBALL, BRUCE - Collaborator; White, Jeffrey; OTTMAN, MICHAEL - University Of Arizona; Wall, Gerard - Gary; Bernacchi, Carl; Morgan, Jack

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/6/2014
Publication Date: 1/19/2015
Citation: Kimball, B.A., White, J.W., Ottman, M.J., Wall, G.W., Bernacchi, C.J., Morgan, J.A. 2015. Predicting canopy temperatures and infrared heater energy requirements for warming field plots. Agronomy Journal. 107:129-141.

Interpretive Summary: In order to study the likely effects of global warming on future ecosystems, including agricultural fields, a method for applying a heating treatment to open-field plant canopies [i.e., a temperature free-air controlled enhancement (T-FACE) system] is needed which will warm vegetation as expected by the future climate. One method which shows promise is warming with infrared (IR) heaters deployed above the experimental plots in a hexagonal pattern. Experiments using such IR heaters have been conducted on wheat a Maricopa, AZ and are underway on soybean at Urbana, IL and prairie at Cheyenne, WY. This paper presents data demonstrating the good performance of these heater systems and also presents theory which can be used to predict the performance and costs of future needed experiments at higher degrees of warming (~10°C) than the present experiment that were conducted with 3.5°C or less of warming. This research will benefit all consumers of food and fiber.

Technical Abstract: Warming open-field plots using arrays of infrared heaters has proven feasible for conducting experiments to determine the likely effects of global warming on various ecosystems. To date, however, such experiments have been done for only a few degrees (= 3.5°C) of warming, yet climate projections, especially for high latitudes, indicate future warming may be 10°C or more. Therefore, there is a need to conduct these experiments with more heating, which increases expense. To estimate energy requirements and costs for such T-FACE (temperature free-air controlled enhancement) experiments, improved theory is presented whereby: (a) the canopy temperature of an unheated plot is computed using the well-accepted Monin-Obukhov Similarity Theory (MOST) to calculate aerodynamic resistance, (b) the desired degrees of warming are added, and (c) the energy balance is re-solved to obtain the additional infrared radiation needed from the heaters to obtain the desired temperature of the heated plots. However, convergence problems were encountered using MOST, which we minimized by using the formulation from Mahrt and Ek (1984) to provide initial values for the iterations. Using MOST also yielded unrealistically cold canopy temperatures under stable, low-wind conditions. Use of a modified natural convection equation for a semi-infinite plane from the American Society of Heating, Refrigerating, and Air Conditioning Engineers (1972) improved agreement with measured canopy temperatures under these conditions. Performance data are presented from T-FACE experiments with 3-m-diameter plots conducted over six wheat crops at Maricopa, Arizona, USA and for one-week periods over soybean at Urbana, Illinois, USA and over northern mixed-grass prairie near Cheyenne, Wyoming, USA. Over the six growing seasons, the T-FACE system over wheat provided degrees of warming such that the modes of the distribution of the warming for day and night were within 0.1°C of the desired setpoint differences of 1.5 and 3.0°C, respectively, but the distributions were skewed to lower degrees of warming because the capacity of the T-FACE system could not maintain the warming under high wind speeds, so the resultant average degrees of warming were 1.3 and 2.7°C during day and night, respectively. The energy requirements of the T-FACE system for raising the wheat canopy temperatures averaged about 7.7 kWh m-2 per day whether determined from the control signals from the dataloggers or predicted using the hybrid Mahrt&Ek/MOST procedure. However, the power company’s meters suggest it could have been about 50% higher. Predictions of canopy temperatures and of infrared heating requirements using the hybrid Mahrt&EK/MOST method agreed with measurements most of the time for wheat, soybean, and prairie, but additional experiments are warranted to test the procedure at higher degrees of warming than 3.5°C, the highest used in experiments to date.