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

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 that future warming may be 10°C or more. Therefore, there is a need to conduct such experiments with more heating, which increases expense. To estimate energy requirements and costs for such temperature free-air controlled enhancement (T-FACE) experiments, improved theory was developed whereby: (i) the canopy temperature of an unheated plot is computed using the well-accepted Monin-Obukhov similarity theory, with some constraints to calculate aerodynamic resistance; (ii) the desired amount of warming is added; and (iii) the energy balance is re-solved to obtain the additional infrared radiation needed from the heaters to attain the desired temperature of the heated plots. Performance data are presented from T-FACE experiments with 3-m-diameter plots conducted over six wheat (Triticum aestivum L.) crops and for 1-wk periods over soybean [Glycine max (L.) Merr.] and northern mixed-grass prairie. The T-FACE system over wheat provided warming temperatures for day and night that were within 0.1°C of the desired setpoint differences. The measured or predicted energy requirements of the T-FACE system for raising the wheat canopy temperatures averaged about 7.0 kWh m^-2 d^-1. Predictions of canopy temperatures and infrared heating requirements agreed with measurements most of the time for wheat, soybean, and prairie.