TDR Soil Moisture Measurements

*** Use of instrument and interpretation of waveforms requires training ***

1) Description:

Time-Domain Reflectometry (TDR) is a technology that was initially developed to allow service personnel to locate damage in buried communication cables. A microwave signal is applied to the coaxial cable and reflected by discontinuities (breaks or shorts) in the cable back to the source, where a video display converts the time delay to distance and graphs a profile of the cable. The apparent distance to the point of damage can be measured and service personnel instructed where to dig. It was found that the apparent distance to the cable break varied from the actual distance depending on the dielectric constant of the surrounding soil, which is directly proportional to soil moisture content.

Soil scientists began using TDR technology in reverse, employing wave guides (broken cables) of known length to deduce soil moisture via several polynomial equations. These wave guides are generally constructed from stainless steel welding rods, which once installed, can be left in place indefinitely. This allows for precise, repetitive, safe, and relatively non-destructive soil moisture measurement at almost any sampling frequency. There are now instruments on the market designed specifically for soil moisture measurement, e.g. Soilmoisture Equipment Corporation (SEC) Trase instrument, which employ automated waveform interpretation, etc. We generally prefer to use the Tektronix Corporation 1502 series instruments, which were designed for cable testing, due to their record of field durability and the amenability for operator interpretation. This interpretation, however, requires a certain amount of operator training to minimize subjectivity and maximize comparability of measurements taken by different operators.

*** Be certain that you are adept at the TDR procedure before taking measurements ***

2) Project Specific Notes:

There are several methods for the construction and installation of wave guides. Some studies employ vertically oriented rod pairs, 5 cm apart, to which test leads are attached directly. This method integrates soil moisture content from soil surface to the lower depth of the rods. On some studies, currently the gradient plots, the TDR rods are oriented parallel to soil horizon and connected to coaxial cable extending above ground. This method, which allows measurement of discrete soil horizons, requires more intensive installation procedures and detailed waveform interpretation. Currently, soil moisture measurements utilizing the TDR technique are made bi-weekly on the Gradient and Riparian projects.

3) Sampling Equipment:

a) Tektronix 1502 Cable Tester with charged battery pack
b) spare battery pack
c) coax to twin-lead test cable with balun Test cable construction from Tektronic to TDR rod or buried cable:
(1) in-line surge surpressor
(2) RF cable assembly 6 ft., RG-59 BNC, 75 ohm coaxial cable with BNC connectors for general-purpose communications and test instrument applications
(3) TV/RF adapter Adapts male BNC to fit female F-type jack
(4) Indoor/Outdoor matching transformer 75 ohm coax/300 ohm twin lead for connecting 75 ohm downlead to 300 ohm antenna
(5) two alligator clips
d) pack with spare test cable and parts
e) field notebook and pencil

4) Sampling Procedure:

a) pull on Power button on lower right hand side of instrument panel
b) be certain Vp is set at .99c (2 knobs to left of power button full clockwise)
c) set distance to meters in setup menu
d) Noise Filter should be one setting clockwise from "HORIZ"
e) Dist/Div setting should be ~0.5 m
f) connect test cable to instrument panel
g) move cursor ("<> position" knob) to the right until end-of-cable inflection is on left side of screen
h) set zero at end of cable(s); determine end of test cable by shorting lead by touching alligator clips together; when measuring horizontally installed waveguides with buried cable, attach one clip to inner lead of coaxial connector and the other to outer (shielding) lead, determine end of buried cable by change in waveform. Place cursor at end of cable(s) point; turn Noise filter knob one notch clockwise; distance display should now read "0.00m".
i) attach clips to TDR rods for vertically installed rods
j) move cursor to endpoint inflection determined by observing change in waveforms
k) read and record apparent distance
l) continue with measurements; machine must be reset if turned off
m) turn instrument off when driving between sites
n) plug in for recharge when finished
o) for the riparian study, the tdr values need to be entered in the "riptdr.xcl" file.

Data from the TDR measurements are converted to dielectric constant from the square of apparent length divided by actual length of waveguide, then to percent soil moisture via a polynomial equation:

% H20 = [-5.3 x 10e-2] + [(2.92 x 10e-2) (D)] - [(5.5 x 10e-4)(De2)] + [(4.3 x 10e-6)(De3)]

where: D = dielectric constant e = exponent