206
Robert B. Jackson, Laurel J. Anderson, and William T. Pockman
et al. 1990, Dirksen and Dasberg 1993) have been
proposed. Relationships for specific soil types such
as organics, minerals, and peats have also been suggested (e.g., Roth et al. 1992). Depending on the
goals and soils in each study, these published
curves may be appropriate (Zegelin et al. 1992).
However, local calibration curves significantly improve estimates of soil moisture (e.g., Dasberg and
Hopmans 1992; Gray and Spies 1995). Despite
claims of TDR manufacturers whose instruments
output Elv using universal curves, a calibration for
each soil should be generated by the user. Calibrations are done using packed soil columns or field
soil cores.
Raw TDR output is a wave trace showing voltage changes in the electromagnetic wave as it reflects back to the instrument (see Fig. 13.3). These
voltage changes correspond to variations in impedance, defined as the total opposition to current
flow in an electrical circuit (Parker 1983). Mismatches in impedance between different materials
along the TDR circuit path cause part of the signal
energy to reflect back. Strong reflections occur at
the soil surface and probe end, showing as peaks
on the wave trace (Fellner-Feldegg 1969, Cassel et
al. 1994). The distance from the soil surface peak
to the final upward inflection represents the travel
time of the electromagnetic wave t, used to calculate Ka. Early TDR required manual measurement
of this distance on the wave trace (e.g., Topp et al.
1982), but many instruments now do this automatically using computer algorithms that search for
characteristic changes in slope (e.g., Baker and Allmaras 1990).
Wave traces can vary substantially from the
shape in Figure 13.3, making identification of the
soil surface or probe end difficult. In highly conductive media, such as clays or saline soils, the
electromagnetic pulse dissipates such that the final
reflection is weak, called "signal attenuation" (see
Fig. 13.3). While there are instrument and probe
modifications that can address this problem, TDR
is limited in very wet, conductive soils (Zegelin et
al. 1992).
A wide variety of TDR probes (or wave guides)
can be purchased or built. The most basic design
includes a connection to conduct current from the
coaxial cable into a set of stainless steel rods in the
soil and a mechanism for holding the rods parallel.
Probes generally range from 10 cm to 2 m in length
and may be inserted into the soil at any angle (Topp
and Davis 1985). They can be installed permanently or moved from site to site. Hook et al. (1992)
created probes with diodes in the handles that
switch from open to short circuits during a measurement. A shorted diode produces a strong reflection, but allows signal energy to pass into the
soil when open. This identifies the soil surface
without reducing signal strength. These workers
also used diodes to divide long probes into segments, allowing measurements of moisture at different depths from one probe. For simple probe designs see Topp et al. (1984), Zegelin et al. (1989),
and Heimovaara (1993).
Advantages of TDR include minimal soil disturbance and rapid, reliable measurements. Some
probes can be built relatively cheaply. While TDR
measures only the soil adjacent to the probe rods
(Baker and Lascano 1989), Elv is averaged over the
probe length, providing an integrated measurement.
Multidiode probes allow simultaneous sampling of
moisture in different soil layers, which could give
important insights into patterns of water flow and
storage in ecosystems. Multiple probes can be connected to a data logger to allow simultaneous, regular readings at many sites (Baker and Allmaras
1990, Herkelrath et al. 1991).
Limitations of TDR include probe installation
problems in stony or shallow soils. Signal attenuation in conductive soils may limit the probe
lengths that can be used. Air pockets may form next
to probes in soils that crack as they dry, causing
measurement errors (Hokett et al. 1992). Sharply
layered soils or large changes in the water profile
with depth can create unexpected reflections in the
wave trace, complicating interpretation (Nadler et
al. 1991). Due to the variability of waveforms that
can be generated in the field, TDR requires training
and practice to use with confidence. Companies that
sell TDR instrumentation include Environmental
Sensors, Inc. (San Diego, CA), Soil Moisture
Equipment Corporation (Goleta, CA), and Tektronix, Inc. (Redmond, OR). Detailed reviews ofTDR
are given in Zegelin et al. (1992) and Cassel et al.
(1994).
Remotely Sensed Data Using
Microwave Radiometers
Remotely sensed data for estimating soil moisture
have been used for several decades (e.g., Schmugge
et al. 1974). Existing methods can be separated into
Robert B. Jackson, Laurel J. Anderson, and William T. Pockman
et al. 1990, Dirksen and Dasberg 1993) have been
proposed. Relationships for specific soil types such
as organics, minerals, and peats have also been suggested (e.g., Roth et al. 1992). Depending on the
goals and soils in each study, these published
curves may be appropriate (Zegelin et al. 1992).
However, local calibration curves significantly improve estimates of soil moisture (e.g., Dasberg and
Hopmans 1992; Gray and Spies 1995). Despite
claims of TDR manufacturers whose instruments
output Elv using universal curves, a calibration for
each soil should be generated by the user. Calibrations are done using packed soil columns or field
soil cores.
Raw TDR output is a wave trace showing voltage changes in the electromagnetic wave as it reflects back to the instrument (see Fig. 13.3). These
voltage changes correspond to variations in impedance, defined as the total opposition to current
flow in an electrical circuit (Parker 1983). Mismatches in impedance between different materials
along the TDR circuit path cause part of the signal
energy to reflect back. Strong reflections occur at
the soil surface and probe end, showing as peaks
on the wave trace (Fellner-Feldegg 1969, Cassel et
al. 1994). The distance from the soil surface peak
to the final upward inflection represents the travel
time of the electromagnetic wave t, used to calculate Ka. Early TDR required manual measurement
of this distance on the wave trace (e.g., Topp et al.
1982), but many instruments now do this automatically using computer algorithms that search for
characteristic changes in slope (e.g., Baker and Allmaras 1990).
Wave traces can vary substantially from the
shape in Figure 13.3, making identification of the
soil surface or probe end difficult. In highly conductive media, such as clays or saline soils, the
electromagnetic pulse dissipates such that the final
reflection is weak, called "signal attenuation" (see
Fig. 13.3). While there are instrument and probe
modifications that can address this problem, TDR
is limited in very wet, conductive soils (Zegelin et
al. 1992).
A wide variety of TDR probes (or wave guides)
can be purchased or built. The most basic design
includes a connection to conduct current from the
coaxial cable into a set of stainless steel rods in the
soil and a mechanism for holding the rods parallel.
Probes generally range from 10 cm to 2 m in length
and may be inserted into the soil at any angle (Topp
and Davis 1985). They can be installed permanently or moved from site to site. Hook et al. (1992)
created probes with diodes in the handles that
switch from open to short circuits during a measurement. A shorted diode produces a strong reflection, but allows signal energy to pass into the
soil when open. This identifies the soil surface
without reducing signal strength. These workers
also used diodes to divide long probes into segments, allowing measurements of moisture at different depths from one probe. For simple probe designs see Topp et al. (1984), Zegelin et al. (1989),
and Heimovaara (1993).
Advantages of TDR include minimal soil disturbance and rapid, reliable measurements. Some
probes can be built relatively cheaply. While TDR
measures only the soil adjacent to the probe rods
(Baker and Lascano 1989), Elv is averaged over the
probe length, providing an integrated measurement.
Multidiode probes allow simultaneous sampling of
moisture in different soil layers, which could give
important insights into patterns of water flow and
storage in ecosystems. Multiple probes can be connected to a data logger to allow simultaneous, regular readings at many sites (Baker and Allmaras
1990, Herkelrath et al. 1991).
Limitations of TDR include probe installation
problems in stony or shallow soils. Signal attenuation in conductive soils may limit the probe
lengths that can be used. Air pockets may form next
to probes in soils that crack as they dry, causing
measurement errors (Hokett et al. 1992). Sharply
layered soils or large changes in the water profile
with depth can create unexpected reflections in the
wave trace, complicating interpretation (Nadler et
al. 1991). Due to the variability of waveforms that
can be generated in the field, TDR requires training
and practice to use with confidence. Companies that
sell TDR instrumentation include Environmental
Sensors, Inc. (San Diego, CA), Soil Moisture
Equipment Corporation (Goleta, CA), and Tektronix, Inc. (Redmond, OR). Detailed reviews ofTDR
are given in Zegelin et al. (1992) and Cassel et al.
(1994).
Remotely Sensed Data Using
Microwave Radiometers
Remotely sensed data for estimating soil moisture
have been used for several decades (e.g., Schmugge
et al. 1974). Existing methods can be separated into