ON THE MEASUREMENT OF SOIL SURFACE-TEMPERATURE

The temperature of the soil surface strongly influences physical, chemical, and biological processes at the surface. Field experiments were conducted to evaluate the accuracy and utility of several different methods for measuring soil surface temperature. Surface temperature of a bare soil was measured simultaneously with infrared thermometers (IRT), bare fine-wire thermocouples (BFT), and soil-encapsulated thermocouples (SET). The SET sensors consisted of fine-wire thermocouples that had been encapsulated with soil from the measurement site of interest. Experiments were conducted on soils with silt loam and coarse sand textures. Continuous measurements were made throughout several drying cycles to test each measurement technique across a range of soil water and environmental conditions. All IRT measurements were corrected for surface emissivity and reflected longwave radiation. During the silt loam trials, both the SET and BFT measurements were lower than the IRT measurements during the day and greater than the IRT readings at night. Results suggest that both the SET and BFT sensors were measuring soil temperature below the immediate surface. On the average, however, the SET and BFT sensors were within +/- 0.8 and +/- 0.5-degrees-C of the IRTs, respectively. During the coarse sand trials, the SET and BFT measurements were consistently greater than the IRT measurements during both day and night periods. Results indicate that the low emissivity of the sand made it difficult to adequately correct the IRT measurements for emissivity and surrounding longwave radiation. During both silt loam and sand trials, the BFT and IRT measurements were in better agreement than the SET and IRT measurements. Nevertheless, data from a field study indicate that the SET sensors are reasonably accurate, and are capable of supplying continuous, long-term data, despite irrigation and rain.