Page 153 - Biosystems Engineering

132 Cha pte r F o u r

wind speed, and stability. Thunholm (1988) tested four methods for
estimating soil thermal conductivities at six locations in Sweden by
using analytical and numerical solutions to the Fourier equation based on
annual temperature variations. The analytical methods produced better
results under ice and snow conditions, insofar as it was difficult to intro-
duce variations in conductivity with depth in numerical solutions.
Nassar and Horton (1989) applied analytical methods for the indi-
rect determination of thermal diffusivity in uniform soils to nonuniform
soils, and demonstrated that these methods were not valid for nonuni-
form soils. After that, Nassar and Horton (1990) proposed a new method
that could be used in nonuniform soils based on a harmonic analysis
obtained by Fourier series. The result was a single overall diffusivity
value for the whole soil layer studied. Conversely, Hurley and Wiltshire
(1993) proposed changes in the expressions for bare and uniform soils
that allowed for the extension of the use of such expressions to soils
characterized by the variation of thermal properties with depth.
Soil thermal properties have been widely studied in terms of
moisture content, texture and structure. Auvermann et al. (1992) pre-
sented a method for determining the thermal characteristics (specific
heat and effective thermal diffusivity) of soils with heterogeneous
moisture contents. This method was based on an explicit finite-
difference model of the one-dimensional heat equation, and the
Lettau regression procedure was used to obtain thermal diffusivity.
Results were faithful to reality and were valid for 24-h temperature
measurements and simulations.
The influence of soil structure on soil thermal properties was ana-
lyzed by Kaune et al. (1993). Soil structure was altered due to mechani-
cal action and irrigation and drying periods. To determine soil thermal
properties, the authors used indirect methods based on harmonic anal-
ysis. They found that effective thermal conductivity and effective ther-
mal diffusivity were lower in soils with altered structures as compared
to undisturbed soils, whereas the values of specific heat were similar.
Noborio and McInnes (1993) studied the influence of soil texture on
effective thermal conductivity. They concluded that clay–solution
interactions significantly affected both properties because of the micro-
structural changes in clay soils. Lipiec et al. (2007) assessed the effects
of tilled- and grass-covered soil on the spatial distribution of the ther-
mal properties in the vineyard interrow with consideration of areas
corresponding to machinery traffic. The mean values of thermal conduc-
tivity obtained were generally greater under tilled- than grass-covered
moist soil and the inverse was true in drier soil.
The above authors used indirect methods to determine soil thermal
properties from ambient temperature variations. Other authors used
temperature variations caused by artificial heat input. Kasubuchi
(1977) developed a twin transient-state cylindrical-probe method for
determining soil thermal conductivity. He applied heat to the soil
using a heating cable and then measured the temperature variations