Comparison of the thermal properties of four wettable and four water‐repellent soils

Water repellency is a widespread phenomenon, but its effect on most physical soil properties is still unknown. The present study deals with the thermal conductivity and volumetric heat capacity of wettable and water‐repellent (hydrophobic) sandy and silty soils. Thermal conductivity and volumetric heat capacity were determined by heat pulse measurements. Heat pulse measurements were made on samples of eight soils (four pairs of wettable soil and the corresponding water‐repellent counterpart). Water repellency either was caused by soil organic matter showing natural repellency or induced by chemical treatment of wettable soils with dichlorodimethylsilane (C2H6SiCl2). Thermal conductivity was also predicted with the models of de Vries and of Campbell. Almost all measured conductivities were larger than those predicted by the de Vries model. For the wettable soils, most of the conductivities in the soil water saturation range between 0.20 and 0.75 differed by more than 0.5 W m−1 K−1 The hydrophobic soils however, showed only in the range around a saturation degree of 0.20 to 0.50 values that deviated more than 0.5 W m−1 K−1 from the predictions. The Campbell model underestimated the conductivity at low saturation for wettable and hydrophobic soil, but overestimated it at high saturation for the wettable soil. Thermal conductivity for either dry or water‐saturated soil was predicted satisfactorily by both models. It was found further that soil wettability had no systematic impact on heat capacity. It is concluded that soil thermal conductivity decreased as soil hydrophobicity increased, whereas the volumetric heat capacity was not affected by soil wettability.