EXPLANATIONS OF SELECTED PRODUCTS
Rangeland Fire
Danger and Red Flag Warnings - Rangeland Fire Danger ratings and Red
Flag Warnings are issued for all counties in the Sioux Falls forecast area.
A fire danger statement and map are issued daily from
April 1st through October 31st...and as needed during the winter months. If the
rangeland fire danger rating becomes very high or extreme, and/or
red flag criteria are met, the fire danger statement will also explain the
specific weather events which prompted the rating. A red flag warning will be issued only when all
three of the following conditions are met consistently, but not necessarily
continuously for 3 hours; a) sustained one-minute winds at
20 feet are at or above 25 mph; b) minimum relative humidity is at or less than
25 percent; c) temperatures at or greater than 75 degrees F. In addition,
the fuels must be dry enough to burn. A Fire Weather Watch can be
issued before a Red Flag Warning, from 12 to 72 hours in advance.
The inputs into the rangeland fire
danger rating are the weekly percent green data from the EROS data
center, afternoon forecast temperature, wind, humidity and cloud
cover. The issuance of a very high or extreme rangeland fire danger
statement occasionally occurs for the Sioux Falls forecast area during the
spring and fall months. This is due to
plentiful dead or dormant grasses, relatively dry humidity and many days where the
wind is fairly strong. But from late May through early September, the issuance
of a very high or extreme rating becomes rare. This is due to annual
and perennial grasses "greening up", much higher humidity and generally decreased
winds. Of course, a severe drought would make the summer issuance of a
very high or extreme rating more common. The red flag warning/fire weather
watch is a rare occurrence for
the Sioux Falls forecast area. However if one is posted, then any fire has
the potential to become quickly out of control and exhibit critical fire
behavior. In addition, when the rangeland fire danger rating is very high or extreme, open burning
should be postponed.
Haines Index
- Haines Index combines the effects of instability and dry air to determine
conditions in which an existing fire has the potential to spread rapidly.
It should not be used to determine if a fire will start. Since wind is not
a parameter of the Haines Index, it is best used for plume-dominated fires which
can develop extreme surface winds through their own internal heat. The
Haines Index is yet to be tested on wind-driven fires. For the Sioux Falls
area, Haines Index takes into account the difference between the 850mb (5,000
ft) temperature and dew point temperature, thus calculating the amount of low
level moisture in the atmosphere. It also looks at stability, by
subtracting the temperatures from 850 to 700mb (10,000 ft) to calculate a lapse
rate. The Haines Index values are as follows: 2 or 3 - very
low; 4 - low; 5 - moderate; 6 - high. When the value is
5 or 6, the probability of extreme fire behavior, including torching, crowning
and spotting is significantly increased. Critical fire behavior is usually
low, with minimal fire spread when the values are 4 or less. Obviously
with a dry and unstable low level atmosphere, Haines Index values will be
increased.
Observed
and Forecast NFDRS - The National Fire Danger Rating System was
developed to give agencies advanced noticed of increased fire danger for
planning and staffing purposes. It is a scientifically based system which
is applicable anywhere in the country and is generated from 1 pm observations
and forecasts nationwide. The NFDRS relates only to the potential of an
initiating fire, one that spreads without crowning or spotting, through uniform
fuels on a continuous slope. It measures fire only from a containment
standpoint as opposed to full extinction. In addition, the NFDRS
represents near worst-case conditions measured at exposed locations at or near
the peak of the normal burning period. One important item to remember is
that the NFDRS is a broad scale rating, approximately for 100,000 acres.
The observed NFDRS is simply the rating calculated from the observed conditions
at 1 pm (mentioned above) and the forecast NFDRS is the rating forecast for the
next day. Besides the basic fire danger
ratings of low, moderate, high, very high and extreme, the NFDRS calculates
parameters to aid agencies in determining staffing levels, how hot a fire will
burn and spread, ignition component and flame length. One possible outcome
of a high fire danger is an agency may have to ban campfires or prescribed
burning on federal lands. Dead
and Live Fuel Moisture - Simply stated, the fuel moisture content
of a fuel is the amount of water in that fuel, expressed as a percent of the
ovendry weight of that fuel. If the fuel were totally dry, then the fuel
moisture content would be zero percent. That being said, when a fuel has
less than 30 percent moisture content, it is basically a dead fuel and is
treated as such. Live fuels will range from 30 percent moisture content,
to around 300 percent, depending on the plant's stage of growth in a
season. When the fuel moisture content is high, fires ignite poorly, or
not at all, because heat energy has to be used to evaporate and drive water from
the plant before it can burn. When the fuel moisture content is low, fires
start easily and will spread rapidly. All of the heat energy goes directly
into the burning flame itself. Dead fuels respond solely to current
environmental conditions (weather) and are critical in determining fire
potential. The size of the fuel relates how fast the fuel will react
to gains or losses in moisture due to changes in its environment.
Therefore dead fuel moisture is classified by timelag and is defined as the
amount of time it takes a fuel to reach two-third's of its way to equilibrium
with its environment. One-hour timelag fuels are fuels which are less than
1/4 inch in diameter and respond very quickly to changes in their
environment. These fuels will only take about an hour to lose or gain
two-thirds of their equilibrium moisture content of their environment.
This size fuel, if dead, is referred to as "fine dead fuel moisture"
and is the most critical size fuel in starting fires. Moving up in size, a
fuel will lose or gain moisture less rapidly through time. Ten hour fuels
range in diameter from 1/4 inch to 1 inch, 100 hour fuels from 1 inch to 3
inches, and 1,000 hour fuels from 3 inches to 8 inches in diameter. 10,000
hour fuels are greater than 8 inches in diameter. Obviously, the 1,000 and
10,000 hour fuels do not burn easily. However, if they do burn, these size
fuels will generate extreme heat often causing extreme fire behavior
conditions. Fire has been known to smolder underneath snow pack through an
entire winter when 10,000 hour fuels were on fire the summer before. Drought
Monitor - The drought monitor map indicates drought by intensity,
which ranges from D1 (least intense) to D4 (most intense). D0 areas are
drought watch regions. The droughts are classified as to their primary
physical effects where A is agricultural (both crops and livestock), W is water
supplies and F is fire danger. The drought intensity categories are based
on six key indicators and numerous supplementary indicators. The six are
Palmer Drought Index, CPC Soil Moisture Model, Daily Streamflow, Percent of
Normal Precipitation, USDA Topsoil Moisture and the Satellite Vegetation Health
Index. The final drought category tends to be based on the majority of
indicators. Greeness
Maps - Greeness products are derived weekly from the Normalized
Difference Vegetative Index and are processed and provided by the EROS Data Center just
outside Sioux Falls, SD. The final greeness products are in a 1km
resolution and are useful in assessing changes due to seasonal curing and
drought conditions. Visual Greeness portrays
vegetative greeness compared to a very green reference, such as an alfalfa
field. Therefore the resulting image is similar to what one would see from
the air. Relative Greeness details how green
the vegetation is compared to how green it has been historically since
1989. Departure from Average Greeness shows
how green each pixel is compared to its average greeness for the current week of
the year. Palmer
Drought Index - The Palmer Drought Index (PDI) is a very slow
responding index to changes in moisture and is best used to monitor water
supplies (such as ground water) or sub-soil moisture which respond slowly to
precipitation anomalies. Palmer attempts to answer the question "How
much precipitation should have occurred during a given period to have kept the
water resources of the area commensurate with their established use?"
He compares the amount of precipitation that should have occurred, with the
amount that actually did occur to measure the departure of the actual moisture
supply from the "normal" supply. The PDI was developed using
data from western Kansas and central Iowa for the late 1800s and the first half
of the 1900s. It attempts to track the evapotranspiration and recharge of
two soil layers - the upper soil layer (plow depth) and the lower soil layer
(root zone depth). The final output of the PDI expresses departure from
normal precipitation, where positive values indicate a surplus of moisture and
negative values indicate less than normal moisture. Keetch-Byram
Drought Index - The Keetch-Byram Drought Index (KBDI) was designed
specifically for fire potential assessment and is a number representing the net
effect of evapotranspiration and precipitation in producing cumulative moisture
deficiency in deep duff and upper soil layers. Since the KBDI was
developed to represent fire developing problems, it responds on a reasonable
time-scale. The KBDI assumes the soil has a field capacity of eight inches
of water, therefore the index is on a scale ranging from 0 to 800. Zero is
the point of no moisture deficiency and 800 is the maximum drought that is
possible. The index number indicates the amount of net rainfall, in
hundredths of an inch, that is required to reduce the index to zero
(saturation). Limitations of the KBDI assume a full recharge of water
during the winter. Also, cool temps and short summers in the northern
climates may keep the KBDI low. Therefore, fire occurrence compared to
KBDI often gives mixed reliability. Crop
Moisture Index - The Crop Moisture Index (CMI) is a derivative of
the PDI and is designed to monitor short-term moisture conditions across
major crop producing regions. The PDI measures long-term moisture
conditions. The CMI is based on the average temperature and total
precipitation for each week within a climate division, as well as the CMI value
from the previous week. The CMI responds rapidly to changing conditions
and is not a good long-term drought monitoring tool. A limitation of the
CMI is that it begins and ends each growing season near zero, which prevents the
CMI from being used to monitor moisture conditions outside the general growing
season. Standardized
Precipitation Index - The Standardized Precipitation Index (SPI)
was designed to quantify the precipitation for multiple time scales. This
is advantageous, in that soil and water needs differ in how quickly they respond
to changing drought or moist conditions. For instance, top soil responds
quickly to moisture or a lack of it, while changes in groundwater reflect
long-term precipitation anomalies. The SPI is calculated for any location
and is based on the long-term precipitation record for a desired period.
This long-term precipitation record is normalized so positive SPI values show
greater than median precipitation and negative values indicate less than median
precipitation. This index is widely used by water management specialists.
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