Evaluation of soil thermal diffusivity algorithms at two equatorial sites in West Africa

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Taofeek Abiodun Otunla
Ezekiel Oluyemi Oladiran


This study presents comparisons between six algorithms used in the calculation of apparent thermal diffusivity (Kh) of the topsoil during measurement campaigns conducted at two equatorial sites. It further investigates the effects of transient and seasonal variations in soil moisture content (theta) on the estimation of Kh. The data used comprise soil temperatures (T) measured at depths of 0.05 m and 0.10 m, and theta within the period of transition from the dry season to the wet season at Ile Ife (7.55˚ N, 4.55˚ E), and for the peak of the wet season at Ibadan (7.44˚ N, 3.90˚ E). The thermal diffusivity, Kh, was calculated from six algorithms, of: harmonic, arctangent, logarithmic, amplitude, phase, and conduction-convection. The reliability of these algorithms was tested using their values to model T at a depth of 0.10 m, where direct measurements were available. The algorithms were further evaluated with statistical indices, including the empirical probability distribution function of the differences between the measured and modeled temperatures ([delta capitalized]T). The maximum absolute values of [delta capitalized]T for the six algorithms investigated were: 0.5˚C, 0.5˚C, 0.5˚C, 1˚C, 1˚C and 1˚C, respectively. Kh showed an increasing trend as theta increased from the dry season to the peak of the wet season, with R2 = 0.70 for the harmonic algorithm. The accuracy of all of the algorithms in modeling T reduced with transient variations of theta. The harmonic, arctangent and logarithmic algorithms were the most appropriate for calculating Kh for the region of study. The empirical relation between theta and Kh and the values of Kh obtained in this study can be used to improve the accuracy of meteorological and hydrological models.

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Otunla, T. A. and Oladiran, E. O. (2013) “Evaluation of soil thermal diffusivity algorithms at two equatorial sites in West Africa”, Annals of Geophysics, 56(1), p. R0101. doi: 10.4401/ag-6170.
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