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TITLE. Soil Moisture of Konza Prairie Soil Biodiversity Project: TDR
(Time Domain Reflectometry)

ABSTRACT.  Soil moisture was estimated at 10 pairs of plots on the Konza
Prairie in Kansas using time-domain reflectometry (TDR) technology.  Rain
out shelters were placed over ten of the plots (each adjacent to a control
plot). Five pairs were located at areas of high above ground biodiversity
and the other five pairs were located at areas of contrasting low above
ground biodiversity. (See associated metadata for Soil Biodiversity Project).
The system used consisted of a Tektronics 1502B reflectometer, a Tektronics
SP232 interface module, RG 58 coaxial cable, and a probe that was left
permanently in the soil.  The reflectometer generated an electrical step
voltage pulse which was propagated down the cable to the probe.  The reflected
voltage waveforem was measured, stored, and displayed by the reflectometer
and the Tektronics SP232 interface module.  The waveform is represented as
2-dimensional data array, with the x-axis representing time (which is
converted to distance using the dielectric constant of the cable) and the
y-axis  representing voltage for the given time:  in this system this value
is represented in millirho units (Tektronics 1991).  The length of the
waveform is deteremined by measuring the difference between the soil air
interface (SAI) and the end of the voltage step reflection (RE).  The SAI
value is equivalent to the apex of the first peak in the wave and the RE value
is equivalent to the intersection of tangents "drawn" at (a) the bottom of
the wave and (b) the point of the steepest rise (see Taylor and Seastedt 1992).
The waveform length is then used to estimate the volumetric and gravimetric
moisture of the surrounding soil.


INVESTIGATOR.  Heather Reed & Tim Seastedt, INSTAAR, CB 450, University of
Colorado, Boulder, CO, (303) 492-7303 and (303) 492-0434 or (303) 492-0434,
heather@culter.colorado.edu, tims@culter.colorado.edu.


VARIABLES.  soil moisture, waveforem length, measurement resolution.



KEYWORDS.  TDR (Time Domain Reflectometry), gravimetric soil moisture,
volumetric soil moisture, Konza Prairie, soil biodiversity.


LOCATION.  The surdy sites were on watersheds 20A,20B,20C,1A,1C, and 1D,
on Konza Prairie, KS.


TIMING.  begin 18 May 1999, ongoing during the growing season.	

CITATIONS.  [1] Topp, G.C., J.L. DAvis, and A.P. Annan. 1980. Electromagnectic
determination of soil water content:  Measurements in coaxial transmission
lines. Water Resources Research16:574-582.
[2]Taylor, R., and T.Seastedt. 1992. Using time-domain reflectometry (TDR)
for soil moisture measurement, detailed methods, Niwot Ridge LTER Program 1992.
Institute for Artic and Alpine Research, University of Colorado, Boulder. 14pp. 


COMMENTS.  In the field, the SAI and RE were approximated by hand using the
waveform displayed on the TEktronics 1502B reflectometer monitor.  
Note that double astericsks(**) are used to indicate exponentiation throughout
the remainder of this section.
The equations used for soil moisture estimation were proveded by Tim Seastedt
with the following comments:
Proposed calculation for TDR:  Our volumetric soil moisture data are calculated
assuming an average probe length of 15cm. Any modification of probe lengths
must be documented, lest these numbers be garbage.  Specifically, the formulas
used for calculating soil moisture are described in the Taylor and Seastedt
(1992) manual and are based on measurements of TDR waveform distances (in cm).
The standard equation (Topp et al. 1980, Water Resources Research 16:574-582)
appears to be as good as any in calculating volumetric soil water.  To avoid
any confusion, the exact formulas are repeated here.
COL 8 = dist = waveform length in cm, (RE-SAI);
COL9 = dia = (dist/(2(15)))**2;
COL10 = volumetric soil moisture = -.053 + .0292*dia - .00055*dia**2 +
.0000043*dia**3.

SAI and RE were converted from inches (displayed on the monitor) to cm:
COL5 = 30.48 * COL4;
COL7 = 30.48*COL6.
Two probes were placed at each plot on opposite ends of the plot.

Note as of 7/99 probes have not been calibrated accounting for the negative
values in volumetric soil moisture.

\type

statistical.

\header

COL1. label=study code, type string, units=none, missing value indicator=,
minimum=, maximum=,precision=

COL2. label=location/treatment code, type=string, units=none, missing value
indicator=, minimum=, maximum=, precision=

COL3. label=date(yymmdd), type=integer, units=none, missing value indicator=,
minimum=, maximum=, precision=

COL4. label=SAI measruement, type=real, units=ft, missing value indicator=,
minimum=, maximum=, precision=

COL5. label=SAI, type=real, units=cm, missing value indicator=, minimum=,
maximum=, precision=

COL6. label=RE measurement, type=real, units=ft, missing value indicator=,
minimum=, maximum=, precision=

COL7. label=RE, type=real, units=cm, missing value indicator=, minimum=,
maximum=, precision=

COL8. label=distance, type=real, units=cm, missing value indicator=, minimum=,
maximum=, precision=

COL9. label=dia, type=real, units=cm^2, missing value indicator=, minimum=,
maximum=, precision=

COL10. label=volumetric soil moisture, type=real, units=percent, missing
value indicator=, minimum=0, maximum=, precision=

COL11. label=gravimetric soil moisture, type=real, units=percent, missing
value indicator=, minimum=0, maximum=, precision=

COL12. label=horizontal scale, type=string, units=none, missing value
indicator=, minimum=, maximum=, precision=