Page 148 - Biosystems Engineering

Models for Heat Transfer in Heated Substrates 127

2. Slope: The amount of heat that strikes the soil reaches its max-
imum value when the surface is perpendicular to radiation.
3. Exposure: It varies according to topography, latitude, direc-
tion, and surface microtopography roughness.
4. Soil surface color: Color plays a key role in the absorption and
reflection of solar radiation in that dark colors absorb more
radiation than light colors.
5. Variations in specific heat and thermal conductivity: Particularly
associated to water content and composition. Bulk density and
moisture content are the factors that most strongly influence
heat transfer in soils. Thus, increases in bulk density and mois-
ture content raise temperature because of the increase in soil
thermal conductivity and specific heat. The effect of peat incor-
poration in decreasing soil thermal conductivity and thermal
diffusivity is diminished at low soil moisture and bulk density
(Ekwue et al. 2005). For that reason, peat soils get warm slowly
due to their low thermal conductivity, which results from the
air filling the pores of such soils (de la Plaza et al. 1999).
6. Water content: The temperatures observed at different depths
during a day are strongly dependent on surface phenomena,
and on other factors such as soil water content below the top
layers (Guaraglia et al. 2001).
7. Surface cover: The presence of vegetation, stubble, snow, or
other elements that cover the soil surface inhibits solar radia-
tion from reaching the soil. Even for severely compacted soils,
the soil thermal regime is very sensitive to surface tilling
(Sikora et al. 1990). Tillage in farmlands plays an important
role not only as a soil management tool to improve the plant
root environment but also as a factor that affects the water
cycle and the energy balance in a region. This is because till-
age greatly affects soil moisture and soil temperature in the
unsaturated soil zone near the surface, which acts as an inter-
face between the subsurface soil layer and the boundary of
the atmosphere that is in contact with the surface (Moroizumi
and Horino 2002).

Other physical and chemical processes cause variations in heat
flow in soils. Such processes include, among others, organic matter
mineralization and the wetting and drying of soil colloids. In addition,
we must not forget that biological activity alters the amount of heat in
the soil during the development of its vital processes.
In greenhouse substrates, where environmental conditions differ
from outdoor conditions, other factors must be taken into consideration.
First, the fraction of solar radiation that reaches the greenhouse sub-
strate decreases due to the greenhouse covering material. In addition,