Calibration of soil heat flux sensors

Soil heat flux is difficult to measure accurately and soil heat flux plates are difficult to calibrate. In this research the reference heat flux was calculated from the temperature gradient and independent thermal conductivity measurements. Reference conductivities, as measured by the non-steady state probe method, have an error of about 2%, while the temperature gradient was measured to an accuracy better than 1%. This results in very reliable reference measurements. Compared with this reference, commercially available heat flux plates have significant inaccuracies. The 1 mm thin TNO PU 43 T sensor was the most accurate with an average relative error of only 4%. A promising new technique is the in situ self calibration, as performed by the Huksefluks HFP-01-SC disc sensor. With self calibration this sensor achieves an accuracy of 5% and confers several advantages for field use. The MIDDLETON CN3 and TNO WS 31S sensors had relative errors of about 20%. The ring shaped sensor Huksefluks SH1 gave relatively poor results, because it measured, in fact, the temperature gradient instead of the heat flux. The results of this sensor remained poor after correcting for the thermal conductivity of the sand. For all sensors the same conclusions hold for non-steady state conditions with evaporation. The often used Philip (1961) correction factor is shown to be not very accurate: in only half of our experiments its use decreased the relative measuring error; and in some cases it made it worse. However, the correction remains useful as a tool for designing soil heat flux sensors; also a positive relation exists between the magnitude of the correction and the inaccuracy of the measurement.