Thursday, August 6, 2009 - 1:50 PM

COS 101-2: Infrared warming of open-field plots from scales of 1 to 100 m

Bruce A. Kimball1, Matthew M. Conley1, and Keith F. Lewin2. (1) U.S. Arid-Land Agricultural Research Center, (2) Brookhaven National Laboratory


There is a need to study the likely effects of global warming on ecosystems with experimental treatments as representative as possible of future environmental conditions. One approach that shows much promise is the use of hexagonal arrays of infrared heaters to warm canopies of vegetation. This approach is currently being used in several experiments on low-stature plants (< 1 m) in 3-m-diameter plots (and smaller). However, in order to approach more realistic ecosystem scales, including tree forest plots, it is necessary to greatly increase plot size.


By nesting hexagonal arrays in a honeycomb pattern, we show that theoretically excellent uniformity of the downwelling thermal radiation (and consequent warming) can be achieved across the plots. Switching from heaters with a characteristic dimension of 60 mm, such as currently being used, to larger heaters with a characteristic dimension of 1220 mm would increase the theoretical heater radiative efficiency (percentage of input electrical energy emitted as thermal radiation) from about 69% to 82% at a wind speed of 4 m s-1. More importantly, the theoretical geometric efficiency (percentage of thermal radiation that falls within the useable plot area) would increase from 37% to about 60% as plot diameter increases from 3 to 20 m and beyond. The overall efficiency is the product of the radiative and geometric efficiencies, which therefore could change from about 25 to 42% at the 4 m s-1 wind speed as one changes from a single hexagon of the smaller heaters over a 3-m-diameter plot to a honeycomb of the larger heaters over 20-m-diameter and larger plots. Assuming, for example, that 4degrees C is the desired degree of warming, that electricity costs $0.1 per kW-hr, and the vegetation is dormant 5 months of the year and actively transpiring for 7 months of the year, under Konza prairie conditions, annual power costs would be on the order of $7,400 for a 3-m-diameter plot and $210,000 for a 20-m-diameter plot. However, there is an economy of scale such that the costs would be about $1,000 and $500 per m2 for the 3- and 20-m plots, respectively.