Ground Surface Moisture Flux Boundary Conditions - Unsaturated Soil Mechanics in Engineering Practice - page 300

400 27

21

Net radiation

300 Surface temperature

16

200

10

100

5

0

− 1

− 7

-100

0 5 10 15 20

Time of day, h

Figure 6.32 Example of relationship between variation in net radiation and ground surface
temperature.

decreases. Around sunrise (e.g., 6:00 a.m.), solar radiation
starts to reach the ground surface and the net radiation
begins to increase. However, the net radiation is still nega-
tive and so the temperature continues to decrease. At some
point after sunrise (e.g., 7:00 a.m.), enough solar radiation
reaches the ground that the net radiation becomes zero.
This is the time when the minimum temperature occurs.
As the sun moves higher in the sky (e.g., 10:00 a.m.),
increasing solar radiation reaches the ground surface,
the net radiation becomes positive, and the temperature
increases.
Solar radiation peaks at around noon when the sun is at
its highest point in the sky. This is also the time when net
radiation is a maximum value. After noon the sun starts to
move lower in the sky and the net radiation then decreases.
However, as long as the net radiation remains positive, tem-
perature at the ground surface will continue to increase. Net
radiation will again be zero as the sun gets lower in the sky
(e.g., 5:00 p.m.). At this point the temperature stops increas-
ing and the maximum temperature occurs. Once again, net
radiation goes negative and temperature starts to decrease.
This illustrates how net radiation changes throughout a typ-
ical day.
The integration of net radiation throughout a day gives
the average daily net radiation in terms of joules per square
meter per day. The average daily net radiation has a sig-
nificant role in determining the pattern of ground surface
temperatures. Latent energy transfer also affects the ground
surface temperature. Energy is required for the evaporation
of water and the melting of snow. Energy that is absorbed
during the phase change of water is not available to increase
the ground (or water) surface temperature.
The daily net radiation that reaches ground surface con-
stitutes primary information that is required for calculating
potential and actual evaporation. It is preferable to measure
net radiation in the field at a weather station by measuring

the components of radiation. There are a number of instru-
ments that can be used to quantify net radiation.
One of the procedures that can be used to obtain net radi-
ation involves the measurement of the Bowen ratio B along
with the total radiation reaching ground surface. The Bowen
ratio is defined as the ratio of the sensible heat Q h and the
latent heat Q e :

Q h
Q e

B = (6.33)

When the Bowen ratio B is less than 1.0, it means that
a greater proportion of the available energy at the ground
surface is passing to the atmosphere as latent heat than as
sensible heat. When latent heating approaches zero, as is
the case in arid regions, the Bowen ratio is a poor choice
of instrument for determining net radiation. For arid regions
it is better to compute the evaporative fraction E v F , which
can be written as

Q e
Q e +

E v F = (6.34)

Q h

6.3.16 Estimation of Net Radiation
Limited attention seems to have been historically given to
making routine measurements of net radiation when measur-
ing weather-related variables. Lack of measurements of net
radiation is mainly due to financial reasons. There are also
theoretical reasons why other approaches have been used
as an indirect means of obtaining values for net radiation.
Net radiation can be estimated based on a number of mete-
orological parameters such as total solar radiation R s ,air
temperature, and other variables, including relative humid-
ity and extraterrestrial radiation R a (Reddy, 1971; Wright,
1982; Dong et al., 1992). Several procedures have been
proposed for the determination of net radiation and a few
methodologies are mentioned below.