Agriculture Reference
In-Depth Information
Direct radiation
Diffuse radiation
(from the
atmosphere)
Reflected radiation
Longwave reradiation
Radiation energy
balance and heat fluxes
Figure 3.8
J
A
Soil
at the soil surface
surface
(redrawn from Linacre
1992).
J
H
Soil Temperature
3.5
Energy Fluxes and Temperature
The energy balance at the soil surface primarily controls soil temperature. The
main input of energy is solar radiation of which a fraction is directly reflected, de-
pending on the reflectance or
albedo
of the surface. Some of the shortwave radi-
ation that is absorbed by the soil surface and vegetation is reradiated to the atmo-
sphere as longwave radiation. Of the remainder, called the
net solar radiation R
N
,
the greatest part is dissipated by evaporation of water (section 6.4.2). The balance
of the absorbed radiation is partitioned between the transfer of sensible heat to
the air immediately above the ground (
J
A
) and the transfer of heat deeper into the
soil (the soil heat flux
J
H
). A summary of the energy fluxes at the surface is shown
in figure 3.8.
The size of
J
H
determines the effect of radiation on soil temperature. The
value of
R
N
, and hence
J
H
, varies with the surface albedo, which depends on these
factors: the nature and color of the surface, the type of vegetation, the latitude,
and the slope of the land in relation to the sun. Representative values of albedo
for different surfaces are given in table 3.4.
3.5.1
Representative Values of Albedo for Different Surfaces
Table 3.4
Range of Albedo
Median Value
Type of Surface
(%)
(%)
Tall grass (2 m) to short grass (0.3 m)
21-25
23
Field crops
15-24
20
Forests
10-18
15
Wet soil to dry soil
11-18
14
Water
4-13
7
Source:
After Linacre (1992)
