Thermal soil properties for vineyard (EFEDA-I) and savanna (HAPEX-Sahel) sites

This paper describes the course of topsoil thermal conductivity, lambda, diffusivity, alpha, and heat capacity, C-h, during two measurement campaigns, conducted in semi-arid areas-the EFEDA-I experiment and HAPEX-Sahel; For the derivation of alpha, five methods (the Amplitude, Phase, Arctangent, Logarithmic and Harmonic equation) were compared. Values of C-h were derived from measurements of soil moisture content, theta, and dry bulk density. lambda was either measured directly (the non-stationary probe method) or calculated from A = alpha . C-h. Thermal soil properties were clearly related to theta (and thus rainfall) throughout the measurement campaigns: hardly any changes occurred during the EFEDA-I experiment where continuous dry conditions prevailed, whereas for HAPEX-Sahel a clear decrease in all thermal properties was observed after the last rainfall. For calculation of alpha, the Amplitude and the Harmonic equation gave the best results. Calculation of alpha and lambda beneath the vegetation plots yielded unreliable results, mainly due to shading effects causing more than-one temperature maximum. Direct measurement of lambda yielded unrealistically low values for the dry soil conditions as encountered in Spain, due to poor contact between probe and soil, A correction for the contact resistance is necessary to obtain better estimates. For HAPEX-Sahel, measured and calculated lambda values were much closer, mainly for reasons of higher theta values and a high soil compaction,which ensured better contact between soil and probes.