Thermal properties in relation to soil water status in sloping vineyard

*** Summary. Knowledge of thermal properties of soil helps in estimating heat fluxes as an important component of heat balance. The research was conducted to evaluate spatial distribution of the soil thermal properties (thermal conductivity, heat capacity and thermal diffusivity) in relation to soil wetness and bulk density in a sloping vineyard under two management systems: cultivated (C) and grass-covered (G) soil. Soil samples were taken in spring and autumn in places corresponding to upper rut (UR), inter-rut (IR) and lower rut (LR) areas, and following determination of current water content they were adjusted to the wetness statuses: dry, field capacity (pF 2.0) and saturated (pF 0). Current soil water content (at sampling) was near field capacity in spring and considerably lower in autumn. Soil water content and bulk density at each soil wetness status, together with soil temperature and texture data, were used for determination of the thermal properties. Thermal conductivity was calculated by the physical-statistical model of Usowicz, and heat capacity – with formulae of de Vries, and thermal diffusivity from the ratio of thermal conductivity and heat capacity. Thermal conductivity and heat capacity increased with increasing water content. Increase of thermal conductivity was greater up to field water capacity than at higher water contents, whereas that of heat capacity was uniform in the whole range of water contents studied. However, thermal diffusivity reached its maximum at and near field water capacity. In autumn, the thermal diffusivity at current water content was slightly lower than at field water capacity, despite appreciably lower current soil water content. This was a resultant effect of water content and bulk density on diffusivity. At both management systems the courses of thermal diffusivity as affected by soil water statuses were similar. The whole range of water status allowed determining possible values of the soil thermal properties. The dispersion of thermal conductivity and heat capacity was highest and lowest at current and dry wetness statuses, respectively. In spring, the dispersion was lower in inter-rut area than under the ruts. Irrespective of the management system, dispersion of the thermal properties under the ruts was lower in autumn than in spring, whereas in the inter-rut area the inverse was true, likely due to respective effects of tillage operations and traffic during growing season. Our results emphasize the need to include spatial variability of the thermal properties within inter-row area to improve accuracy in evaluating the energy balance in a vineyard.