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Soil Heat Flow under an Orchard Heater¹

ABSTRACT

Soil heat flux measurements were made under and near an auto clean stack orchard heater to estimate the amount of energy being lost. Heat flux plates were buried at the 5-cm depth at 0, 50, and 100 cm from the heater center. The heat flux plate measurements were corrected for heat stored in the upper 5-cm of soil using temperatures measured by four thermocouples in series at depths of 0.0, 2.5, 5.0, 9.0, and 27.0 cm at each location and for water content and bulk density data. The resulting surface heat flux data were spatially averaged over a 100-cm radius circle around the heater. Soil surface temperatures under the heater fluctuated between 232° and 288°C during the heating interval. Soil temperatures at depths > 2.5 cm under the heater did not exceed 75°C. Surface soil temperatures at the 50- and 100-cm distances from the heater center rose from 4.4° and 4.5°C to 18.5° and 8.3°C, respectively. The spatial average soil heat flux increased rapidly to 82 mW/cm² and then rapidly decreased and fluctuated around 13 mW/cm² for the majority of the 39-min heating interval. This energy represents 2% of the total energy portion of the heater output. It was concluded that the energy absorbed by the soil was not effective in frost protection and should be conserved.

¹ Paper no. 5742 of the Journal Series of the Pennsylvania Agric. Exp. Stn. Supported by Northeast Regional Project NE-48. Authorized for publication May 15, 1979.

² Associate Professor of Soil Physics and former Professor of Microclimatology, The Pennsylvania State University, University Park, PA 16802. The junior author is presently Professor of Climatology, Univ. of Florida, Gainesville, FL 32611.

Received for publication May 31, 1979. Accepted for publication August 27, 1979.