20. Soil Temperature and Heat Transport in Soils

Soil temperature varies over a wide range, depending on annual and diurnal cycles of incoming solar energy and on the soil surface properties. Plants can grow when soil root zone temperatures are in the relatively narrow range 0–40 °C. Soil-water hydraulic conductivity and soil-water retention curves are functions of temperature because of the temperature dependence of viscosity, surface tension and the density of water. Characteristic annual and diurnal cycles of soil temperature (at various depths) characterise the soil-temperature regimen. This statistical characteristic is typical of particular soils and depends on climate. The basic soil-heat characteristics (specific soil heat capacity, thermal conductivity) and basic modes of heat transport, namely, conduction, convection and radiation, are defined. The chapter presents governing equations of simultaneous heat and water transport in non-isothermal soil conditions. The influence of soil temperature on basic soil-water transport and retention characteristics are addressed.