THEORETICAL AND FIELD COMPARISON OF 2 TYPES OF SOIL HEAT FLUXMETER

Accurate measurements of soil heat flux are important for energy balan ce studies on bare soils. Measurements are usually made with passive t ransducers that transform the vertical soil heat flux into an e.m.f. M easurement errors or bias result from differences between the calibrat ion coefficients provided by the manufacturer and those determined in the field. These differences result from bad thermal contact between t he soil and the fluxmeter and/or changes in soil thermal conductivity. New printed circuit heat fluxmeters are very thin (0.2 mm) and they p rovide a better thermal contact with the soil because they have an ext ernal copper layer instead of an insulating resin. We carried out a th eoretical analysis to identify properties of the transducers (geometri cal, thermal or electrical) most important for reducing the calibratio n variability, The transducer thickness was found to reduce the calibr ation variability due to large soil thermal conductivity variations. T ransducer thermal conductivity is also important when the soil thermal conductivity is accurately known. The printed circuit transducers and classical soil heat flux transducers (thermopiles) where then compare d in three different soils, a sandy loam, a loamy and a chalky soil un der changing climatic conditions in spring. The outputs of both transd ucers were compared to reference soil heat flux measurements obtained by the heat storage method. The thermopile transducers were more sensi tive (4.1 mu V W-1 m(2)) than the printed circuit transducers (1.6 mu V W-1 m(2)). Both transducers gave similar responses when the soil the rmal conductivity varied over a narrow range. The total variation of t he calibration coefficients of the printed circuit transducer was smal ler for all three soils and for days where the soil thermal conductivi ty varied widely. We conclude that the printed circuit transducers sho uld be used when field calibration is not possible, or when the calibr ation is not stable following large soil thermal conductivity variatio ns. The experiment also showed that the theory does not completely des cribe the interaction between calibration coefficients and soil proper ties. We have therefore developed a new interpretation of the experime ntal data that Lakes into account the thermal contact between the soil and the transducer. (C) 1997 Elsevier Science B.V.