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1B.1
2 cm
5 cm
10 cm
30 cm
50 cm
136.5 cm
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Performance of high temperature heat flux
plates and soil moisture probes during
controlled surface fires
Soil Temperature (oC)
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250
200
150
100
50
W. J. Massman1 , J. M. Frank, S. M. Massman, and
W. D. Shepperd
USDA Forest Service, Rocky Mountain Research
Station, Fort Collins, Colorado
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0
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11
13
15
17
19
21
Day of Year 2002
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25
27
29
Figure 1: Twenty days of soil temperatures at the
slash pile burn site. Time series begin two days before
the fire. The fire was initiated about 12:20 PM MST
on day 11 of 2002 and burned for several hours.
Introduction
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Soil Heat Flux (W m )
Natural and prescribed fires play an important role
in managing and maintaining most ecosystems in the
400
western United States. The high soil temperatures
200
associated with fire influence forests and their abili0
ty to regenerate after a fire by altering soil proper-200
ties and soil chemistry and by killing microbes, plant
-400
roots, and seeds. Because prescribed fire is frequently
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used to reduce surface fuels, it is important to know
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-1000
how fuel conditions, soil moisture, and soil proper-1200
ties interact to determine the soil temperatures, the
5 cm (Thermonetics Corporation)
10 cm (Thermonetics Corporation)
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depth of the soil thermal pulse, and the response of
30 cm (Thermonetics Corporation)
50 cm (Radiation Energy Balance Systems)
136.5 cm (Radiation Energy Balance Systems)
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the soil biota to soil heating. This report presents the
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results of experimental tests of a high temperature soil
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27
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Day of Year 2002
heat flux plate and a high temperature soil moisture
probe. These sensors are intended to provide data be- Figure 2: Twenty days of soil heat fluxes at the slash
fore, during, and after a prescribed burn in support of pile burn site. Time series begin two days before the
long term studies of soil microbial response to fires.
fire. The fire was initiated about 12:20 PM MST on
day 11 of 2002 and burned for several hours. The
manufacturers of the soil heat flux plates used during
2 High Temperature Soil
this experiment are given in the figure legend.
Heat Flux Plates
More details concerning the controlled burn experiments and the soil heat flux and concurrent soil temperature measurements can be found in Massman et
al. (2003). This section summarizes some of their results and report on the post-fire calibration of the high
temperature heat flux sensors.
The high temperature heat flux probes tested in this
experiment are available from Thermonetics Corporation (La Jolla, CA). They were installed, along with
thermocouples (Figure 1), at a slash pile burn site
within Manitou Experimental Forest (southern Colorado) on October 12, 2001 at 0.05, 0.10, and 0.30 m
depths (Figure 2). The slash pile was then constructed by hand above the sensors on October 18, 2001. It
was approximately conical in shape, 6 m high, and 9
m in diameter at the base. The slash pile was burned
on January 11, 2002. The total loading was about 560
tonsM hectare−1 .
Two different methods were used to evaluate sensor
performance. The first involved comparing the observed soil heat flux data to similar data from the only
other comparable experiment we are aware of (Tunstall et al. 1973). The comparisons were favorable
and the results of both experiments agreed. Tunstall
et al. (1973) observed a maximum surface heat flux of
about 2000 W m−2 and the present study suggested
that the maximum surface heat flux was about 2350 W
1 Corresponding author address:
W.J. Massman, USDA/Forest Service, 240 West prospect, Fort Colins, CO 80526;
e-mail: wmassman@fs.fed.us
1
2003 High-Temperature TDR Test, Manitou Experimental Forest, Colorado
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0.20
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Volumetric soil moisture (m3 m-3 )
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Depth (cm)
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-30
-50
0.15
0.10
0.05
model, equation (2)
standard error of the estimate of the model
measured maximum downward heat flux
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0
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600
900
1200
1500
1800
2100
0.00
128
2400
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130
-2
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132
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134
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Day of Year 2003
Soil Heat Flux (W m )
Figure 3: Vertical profile of maximum measured soil
heat fluxes (•) and the corresponding curve fit with
an exponential decay function at the slash pile burn
site. By extrapolation the maximum surface heat flux
was about 2350 W m−2 .
Figure 4: Seven days of soil moisture probe data at
the campfire site. The fire was initiated about 1 pm
(MDT) May 8 (day 128), 2003. The probe began to
fail about day 131, but preformed intermittently on
days 132 and 133. The probe was retrieved on day
134.
m−2 (Figure 3). The second validation test involved
retrieving the heat flux plates and checking their calibration against their original calibration factors. All
post-fire calibrations were within ± 10% of their original values.
400
5 cm
Top of TDR Probe
Side of TDR Probe
Bottom of TDR Probe
Top of Coaxial Cable
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3
Soil Temperature ( oC)
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High Temperature Soil
Moisture Probe
The test of the high temperature soil moisture probe
was performed May of 2003 at a campfire site near
the headquarters of the Manitou Experimental Forest. The sensor was a prototype TDR designed and
fabricated by Zostrich Geotechnical (Ellensburg, WA).
The campfire site was excavated and the stainless steel
needles of the sensor were installed at a nominal depth
of 5 cm. Simultaneous soil and probe body temperature measurements were made with thermocouples.
The hole was backfilled and a cup or so of water was
added to the soil. The fire was initiated about 1 pm
(MDT) on May 8. Fuel was added to the fire at irregular intervals until about 4 pm (MDT) May 9. The
sensor and thermocouples were retrieved on May 14.
Figure 4 shows the soil moisture data obtained during the week of May 8 - 14, 2003 and Figure 5 shows
the concurrent temperatures. The sensor performed
well during the drying portion of the experiment (days
128-130). The early increase in measured soil moisture
is attributed to the downward movement of the water
that had been added to the soil surface just prior to
the construction and initiation of the fire. However,
after day 130 the sensor began to fail and performed
intermittently after that. Subsequent lab examinations suggested that the Teflon coated coaxial cable
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150
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50
0
128
129
130
131
132
133
134
135
Day of Year 2003
Figure 5: Seven days of soil moisture probe and soil
temperature data. The experiment began midway
through day 128 and ended mid way through day 134.
had melted in places and had exposed the cable wires
to the soil (and any remaining soil moisture). Nevertheless, and remarkably, after taping the coaxial cables
with electrical insulating tape the sensor performed in
the lab as expected and it was impossible to cause it to
fail again. According to the temperature data (Figure
5) the sensor began to fail when the top of the coaxial
cable reached about 240 C, which is near (but a bit
below) the 260 C melting point temperature of Teflon.
Figure 6 shows the pre- and post-fire TDR calibration data performed using soils characteristic of the
site. The post-fire calibration data were within ± 0.02
(absolute units of θv ) of the original calibration.
2
High-Temperature TDR Prototype Probe Calibration
0.30
0.20
3
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TDR Measured θv (m m )
0.25
0.15
0.10
0.05
Zostrich Geotechnical, Prototype Probe, Pre-fire
Zostrich Geotechnical, Prototype Probe, Post-fire
Campbell Scientific, Inc., CS610 Probe 1
Campbell Scientific, Inc., CS610 Probe 2
Campbell Scientific, Inc., CS610 Probe 3
0.00
0.00
0.05
0.10
0.15
0.20
3
0.25
0.30
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Oven Dry Measured θv (m m )
Figure 6: Pre- and post-fire calibration of the high
temperature TDR soil moisture probe. The calibration used soils native to the site and the physically
based model of Topp et al. (1980) with empirically
determined coefficients.
4
Conclusions
All results to date suggest that the high temperature
heat flux plates performed reliably.
Present results suggest that the high temperature
soil moisture probe performed very well and possibly better than expected. Post-fire examinations have
suggested a few simple design changes in the probe
and further tests are planned.
References
Massman, W.J., Frank, J.M., Shepperd, W.D., and
Platten, M.J. (2003) In situ soil temperature and
heat flux measurements during controlled surface burns at a southern Colorado forest site.
In: (Eds. Omi, P.N. and L.A. Joyce) Fire, fuel treatments, and ecological restoration: Conference proceedings; 2002 16-18 April; Fort Collins,
CO. USDA Forest Service Proceedings RMRS-P29, 69-87.
Topp, G.C., Davis, J.L., and Annan, A.P. (1980)
Electromagnetic determination of soil water content: measurements in coaxial transmission lines.
Water Resources Research, 16, 574-582.
Tunstall, B.R., Martin, T., Walker, J., Gill, A.M.,
Aston, A. (1976) Soil temperatures induced by
an experimental log pile burn: Preliminary data
analysis. CSIRO Division of Land Use Research,
Technical Memorandum 76/20.
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