Frost Protection - UTL Repository

PASSIVE PROTECTION METHODS
of minerals, organic matter, water and air spaces, which all have different
characteristics. The specific heat of a soil (J kg -1 °C -1 ) is the energy needed to
raise 1 kg of soil by 1 °C (1 K). Multiplying the bulk density by the specific heat
gives the volumetric heat capacity (C V ) in J m -3 °C -1 , which is the energy in
joules needed to raise the temperature of a cubic metre of soil by 1 °C (1 K).
The thermal conductivity (K s ) in W m -1 °C -1 is a factor that relates soil heat
flux density (G) in W m -2 to the temperature gradient in the soil.
⎛ T − T
⎟ ⎞
G = − ⎜ 2 1 Ks W m -2 Eq. 6.1
⎝ z 2 − z 1 ⎠
where T 1 is the temperature at depth z 1 and T 2 is the temperature at depth z 2 , which
is farther from the surface. The minus sign is included to make G positive when the
flux is downward. The thermal conductivity is a measure of how fast heat transfers
through the soil and the heat capacity is a measure of how much energy is needed
to raise the temperature by 1 °C. The diffusivity (κ T ) in m 2 s -1 , which is a measure
of how fast temperature will propagate through the soil, is given by:
κ T
K s
= m 2 s -1 Eq. 6.2
C V
An estimate of the soil surface temperature range (R o ) in °C, for a soil with
uniform properties, is given by:
R o
1
⎡
2
⎛ π ⎞ ⎤
= R z exp⎢z ⎜ ⎟ ⎥ °C Eq. 6.3
⎢
⎣ ⎝ κ T p ⎠ ⎥
⎦
where (R z ) is the temperature range in °C at some depth z in metres and (p) is
the oscillation period in seconds (= 86 400 s per day). For a fixed R z value, R o
decreases as the magnitude of κ T increases. For frost protection, the goal is to
minimize the range of R o , which is accomplished by maximizing κ T . Therefore,
soils with high κ T are less prone to frost damage and the soil water content
should be managed to achieve the highest possible κ T during frost sensitive
periods. Sample soil thermal characteristics for sandy, clay and organic (peat)
soils (Monteith and Unsworth, 1990) are shown in Figure 6.9.
Dark coloured, moist heavy soils tend to absorb more sunlight but have a lower
thermal conductivity than lighter sandy soils (Figure 6.9). Consequently, the
diffusivity is less and they are more prone to frost damage. The heat capacity of
organic (peat) soil changes considerably from less than sand and clay when dry to
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