Environmental Engineering Reference
In-Depth Information
However, significant deviation has been noted in organic soils. Therefore, it may be
necessary to develop soil-specific calibration curves for application of the TDR
method in organic soils typically found in wetlands (see next section for calibration
methods).
TDR wave guides (or probes) can be installed vertically from the surface or
horizontally in a soil pit, which should be refilled very carefully to prevent prefer-
ential infiltration affecting the measurements. The measurement is sensitive to soil
disturbance and gaps between the probe and soil matrix. Therefore, probes must be
inserted straight into the soil, with minimum wobble, to minimize disturbance.
A TDR probe measures an average
θ
v
over the entire probe length in a cylindrical
region within a diameter of approximately 1.5 times the rod separation (Ferr´ and
Topp
2002
). If the depth to the water table is shallow (e.g.,
0.3 m), vertical probes
may be used to cover the entire vadose zone. Where the water table is deeper,
horizontal probes need to be installed at multiple depths to measure a profile of
water content from the surface to the water table.
Probes may be connected to a portable field device for manual recording of wave
forms and determination of
<
ε
r
, or connected to a digital datalogger. Most of
commercially available TDR devices determine
θ
v
using internal algorithms and
output the value of
θ
v
, which is a convenient feature for long-term monitoring.
However, it is prudent to store the raw waveform data and periodically check the
accuracy of automatically determined
θ
v
. Detailed discussion on the TDR method
and useful guidance for its application are found in Ferr
´
and Topp (
2002
).
3.9.3 Capacitance Method
This method also utilizes the large contrast in dielectric permittivity between water
and other soil components, but it is based on the principle that frequency of
oscillation of a circuit consisting of an electrode-soil capacitor is a function of
dielectric permittivity (Starr and Paltineanu
2002
). Since the functional relationship
is dependent on electrode configuration and soil type, soil-specific calibration is
required to calculate
θ
v
from the frequency measured with a capacitance probe.
Compared to the TDR method, which yields reasonably accurate results using the
universal formula, the disadvantage of the capacitance method is the necessity of
soil-specific calibration. On the other hand, once well calibrated, the capacitance
method offers a much more robust and convenient tool for continuous monitoring
of soil water content than the TDR method. Most commercially available capaci-
tance probes are designed to work with standard dataloggers and can be used as part
of the standard collection of sensors that make up hydrological monitoring stations.
This is another advantage over TDR, which typically requires an expensive control
unit in addition to a datalogger.
Depending on the monitoring objectives and probe length, capacitance probes
may be installed vertically from the surface, or horizontally at multiple depths in a
soil pit. Similar to TDR wave guides, measurement with capacitance probes is
