For more details on the burn program at Pleasant Valley Conservancy, and an 18-year summary of burns, see this link.
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This has been a challenging year for us to do burns because of the permitting system under which we have to work. There have been a number of low-humidity days during which we are not able to burn, although folks outside of the state-controlled areas could (and did) burn with impunity.
See the text below this map for a discussion and some comments on burning under low-humidity conditions.
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| This map shows the extensive burn program that was accomplished in spring 2015. For more details on prescribed burns at Pleasant Valley Conservancy, access this link. |
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From the Wisconsin Prescribed Fire Council listserv
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Burning with low humidity
Posted by: jim@quercus-ls.com
jsetd
Date: Wed Apr
15, 2015 6:47 pm ((PDT))
I've been trading e-mails with some fellow prescribed
burners about burning in the recent low-humidity conditions. I'd like to open up the topic to the group,
and maybe add some nuance to the discussion.
I'm taking a little flak about my decisions to burn the
past two days, so I want to give a little background. Why would a prescribed fire practitioner even consider burning when the humidity is
below 20%? Doesn't that violate your prescription? Isn't it irresponsible?
Our prescriptions are based on fire behavior, as
predicted by Behave [U.S. Forest Service computer program] fire behavior models for the fuel type and weather conditions,
rather than limits on individual parameters.
(We do use parameter limits when dictated by the client, such as DNR or
NRCS.)
On Tuesday afternoon we burned two small units. One was about 2 acres, half good tallgrass
(fuel model gr6) and half bluegrass/fescue (fuel model gr3), with a high point
in the center. I measured 75 deg, 25%
rh, and light wind 1-2 mph, with a Kestrel 3000. The unit burned nicely, pulling in to the
high point, and smoke went straight up as desired. The fire was mostly flanking, the head fire
behavior was 15' flames and 120 ft/min rate of spread, well within
prescription. The other unit was less
than a half acre, no more than 50' across at any point, and was burned with
backing and flanking fire with flame lengths under 8'. These units were well
suited to burning with light winds and the fire behavior was by no means
extreme.
I found out later that the Madison airport was measuring
14% rh when I measured 25%. Measurement accuracy is another lengthy topic but I
think that fire behavior is the bottom line.
On Wednesday we had multiple units to burn at a site on
the north side of Lake Mendota. We
finished a couple units in the morning with a minimum humidity reading of
33%. At 13:00 I measured 25% rh, the
Madison airport was reporting 20%. We
burned a 3.5 acre unit with tallgrass (gr6) and cattail/sedge (gr8). The backfire in the gr8 fuel was 8' flames
and 50 ft/min spread rate, headfire was 15' flames and 100 ft/min, again well within prescription.
The next unit was a 9-acre gr6 unit. Measured rh was 27%, Madison airport
reporting 17%. Wind was 4-7 mph. The
crew was thoroughly briefed on the red flag conditions, the low rh readings,
and the observed and expected fire behavior.
We decided to go ahead, and burned the unit with primarily flanking
fire, with 3-8' flames. We did send some
head fire when we had over 100' of black downwind, and had 15-20' flames with
150-180 ft/min spread. That's pretty big
fire but still within prescription and less than we often see in tallgrass with
higher humidity and more wind.
All of the units we burned Tuesday and Wednesday
afternoon had very good firebreaks, with little or no receptive fuel
downwind. I never felt that we were
taking any excessive risks in burning these units, and never felt that the fire
behavior was threatening. I'm not saying
that everyone should blithely go out and burn no matter how low the humidity,
but it is possible with the right burn units to burn responsibly at these
conditions.
Jim Elleson
Quercus Land Stewardship Services
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