Environmental Engineering Reference
In-Depth Information
a temperature set at 105
C for 24-48 h to evaporate all liquid water without
volatilizing organic components of the soil. The sample weight may also be measured
periodically during drying until it does not change any longer. Samples are cooled in
a sealed container with desiccant to prevent absorption of atmospheric vapor during
cooling, and weighed again. Volumetric soil water content,
θ
v
(cm
3
/cm
3
), is given by
θ
v
¼½ð
Þ=
=
original weight
dry weight
density of water
sample volume
(3.43)
The same sample may be used to determine other soil parameters such as dry bulk
The water content determined with this method is commonly used as the
reference to test or calibrate other methods. However, it should be noted that the
thermo-gravimetric method does not necessarily yield exact results (Topp and Ferr´
2002
) because the measured value may be affected by the drying temperature and
time, vapor absorption during cooling, and most importantly, errors in measurement
of sample volume and weight. A major disadvantage of this method is that it
requires the removal of the sample and is not suitable for continuous, in situ
monitoring of soil water. Therefore, instrumental methods are commonly used for
continuous monitoring.
3.9.2 Time Domain Reflectometry
Of the various types of instruments available for continuous monitoring of soil
moisture, time domain reflectometry (TDR) and the capacitance method are the
most widely used (Ferr
´
and Topp
2002
). These methods both make use of the fact
that the velocity of electromagnetic (EM) waves is equal to the speed of light
(
c
10
8
ms
1
) in a vacuum but is lower in other media. EM-wave velocity is
determined by a property called dielectric permittivity. The dielectric permittivity
of water relative to a vacuum is much greater (
80) than that of air (
1) or soil
solids (
¼
3.0
3-8). Therefore, volumetric water content can be estimated from
measurements of soil dielectric permittivity.
In the TDR method, a very sharp voltage pulse from the signal source travels
through the soil along a wave guide, typically consisting of parallel stainless steel
rods (Fig.
3.29
), and is reflected back to the source. The reflected signal is recorded
as a time series of voltage values, commonly called the wave form (Fig.
3.30
), in
which the time to the negative peak (
t
1
) indicates the two-way travel time of the EM
wave between the source and the top of the wave guide, and the time to a rapid rise
(
t
2
) indicates the two-way travel time between the source and the bottom of
the wave guide. If the length of wave guide is
L
(m), then the apparent velocity
of the EM wave (
v
EM
) in the soil is
v
EM
¼
2
L
2
=
ð
t
1
Þ
(3.44)
