Chapter 14: CPS Rx – Fire Modeling

CPS/Rx and Fire Behavior Modeling

TASK 1. Prepare a summary of burn objectives for the Incident Commander. These are a part of the prescription.

TASK 2. Prepare the firing sequence and timing for the burn. The objective is to remove dead and live fuel in the amount specified in the objectives of the burn plan. The firing plan should be written to accomplish the objectives of the prescription.

This is easy to say….. and hard to do.

The authority to ignite the area is conditional: The fire danger must be within the specified range and be predicted to remain there for the duration of the burn.

The Prescriptive Fire Danger Window

The fire danger window is a description of acceptable conditions; fire behavior must be within reasonable boundaries. This specified range of fire danger and behavior elements is called “the prescription window,” or “RX window.”

Flame lengths and rate of spreads are calculated for high, mid, and low points in the window. The fire behavior model data are derived from combinations of values of weather, topography, fuel type and condition. Eighteen fire behavior entries are required on the fire model input worksheet. Use of the fire behavior model, will describe generalized flame lengths and rate of spreads. Additional input can be made to determine the values for HEAD, FLANKING and BACKING fires. These calculated flame lengths and rates of spread are only computations and not derived from on site conditions. They are not to be taken for data that can replace the need for test fires.

Fire Behavior Modeling and its Use in Prescribed Fire Planning

Utilization of behavior models assures that the burn planners have estimated results of the project by using the latest technology available. The results from this torching the land require predictions prior to approval of the plan. These fire behavior models are the most practical and the best method available to initiate the burn planning.

When to Rely Upon the Fire Modeling

It is necessary to know when to use fire behavior models and when such use is not appropriate. Fire behavior modeling is a good planning tool but does not provide adequate specifics to use for tactical actions.

The limitations of the fire model are explained in a paper written by Richard C. Rothermel titled, ‘How to Predict the Spread and Intensity of Forest and Range Fires.’ Published in June of 1983, NFES #1573.

“The fire model is primarily intended to describe a flame front advancing steadily in surface fuels within 6 feet of, and contiguous to, the ground”

Why conduct and evaluate a test burn if the fire behavior modeling can predict the results? Because the prescriptive window is comprised of fire danger ingredients and not those things that cause the fire behavior differences. The fire behavior model predicts the fire intensity based upon fire danger conditions that remain constant over the time frame selected.

The values of weather, slope, aspect and fuel condition in the general area are considered to remain constant during the time of burning. The fire behavior model will produce a constant, single, intensity. Weather elements consisting of, fuel type, fuel moisture and slope provide inputs for the fire behavior model. The calculations indicate that the fire will burn with 10-foot flame lengths from 10:00 a.m. to 4:00 p.m.

The model inputs do not consider variations in fuel temperature within the fuel bed or how fire enters the fuel. These fuel temperature differences would be 30 to 60 degrees. Uneven surface and fuel temperatures determine the air temperature, relative humidity and 10-hour fuel stick. The fire behavior model uses air temperature taken four feet above the bare ground and fuel stick values representing the generalized fuel flammability. This is a fire danger focus. Fuel temperatures are a fire behavior focus.

Model inputs consider the fuel flammability as a stable element for the duration of the time selected. In reality, we know this is not the case. The fuel flammability changes hour by hour. Fuel is not a stable element because its flammability is constantly changing. Fuel on west aspects in the morning, compare very differently from the same fuel flammability in the mid afternoon.

Will the fire produce the flame lengths and consume the predicted amount of dead and live fuels, calculated, by the fire behavior formula? No, it will not! This calculation is theoretical and is not the best information to yield the best job of prescribed burning.

Should we make more calculations then? What if we made more calculations for various combinations within the RX window, until all the variables are put into the model?

Visualize the variations in the topography that are in a typical burn project. In order to secure sufficient outputs of the fire behavior model, calculations would be necessary for each 2-hour period, each different aspect, slope, mid flame wind and so on.

Air temperature and relative humidity readings should be recorded at all varying points of the topography.

Even after all that, the theoretical flame length would only be a better estimate. Obviously, this idea is not practical. The fire behavior model has done its job and the burn plan is approved.

Stop relying upon the fire behavior models to predict fire behavior variations using the fire danger ingredients. When the burn plan is signed and approved, the use for fire modeling is over.

Using Test Burn Observations

Next on the agenda is to complete a test burn. Previously, there was little information available on how to evaluate a test burn. CPS/RX uses real time, non theoretical, test fire observations to capturing fire intensity and consumption information. There is no need to compare the test burn intensities to the models outputs. The test burn should be done to gain real evidence of the intensity variations that can occur within the RX fire danger window.

Test burns are the optimum method to establish the extremes of fire intensity.

The Test Burn

Test burning is highly recommended before staffing on site and committing the whole area to burn. The purpose of the test is to assure the experienced fire officer that the fire will burn within thresholds of control and with enough heat to remove the prescribed amount of vegetation from the land. What does the officer see during the test burn? How are the evaluations made? Most fire officers simply say the test indicates the fire will carry and “burn OK” or it “will not burn OK.” And on that basis the burn is either initiated or delayed. Other personnel involved may not know the processes the fire officer went through to arrive at the conclusion. CPS/RX has attempted to identify and to put into words, the processes used by some of the experienced fire officers.

Important Information Observations should Capture

Selecting where and when to conduct the test. In order to obtain the best data the time and the placement of the test burn should be carefully selected. Find a place on the topography that represents areas where you want to know how the fire will burn. If there are many topographic or fuel variations within the burn area, pick one with some of the variations. Select an area where the fire can freely run, flank and back so the variations of fire alignments can be observed. Time the test burn so that the aspect is at the peak of the flammability period. Obviously, the tests need to be done prior to the day of actual burning unless the project is very small or quite simple.

Before lighting the test burn take a photo of the area to record the before situation. Record the on site weather, time of day, aspect and the hot and cool fuel temperatures as well as the fuel temperature spread. The CPS Test Burn Form is designed for recording written data.

Check to assure that conditions are within prescriptive limits and that holding crews are in place to extinguish the burn on command.

Light the fuel at a single point and allow the fire to spread. Make sure the fire is burning free and is beyond the influence of the other side of the fire before noting the various flame lengths produced at the HEAD, FLANKS and HEEL of the fire. Photograph the burn with a video camera. When the fire has burned long and far enough to gather this data have the crew extinguish the fire.

This is a good training opportunity for firefighters. Setting up a drill helps others to learn how to evaluate wildland fire and predict variations before attacking the fire. Thus, the training benefit is established and quantified.

Evaluate the information obtained by the test burn. What was the flame length and consumption ratio differences on the HEAD, FLANKS and HEEL of the burn? Compare the objective consumption percentage of dead and live fuel consumed with the actual consumption in the various sections of the test burn. If the test burn did not accomplish the objective with the head, flanks or heel of the fire the fire danger is too high or too low. This information would indicate that a change in the RX window is necessary. If the head fire was the only place that enough consumption was attained, the fuel must be more flammable to make the burn a success.

What we are looking for that will be evidence the fire will accomplish the consumption objective is that the fuel objective has been attained between the head and heel of the fire. This is evidence that the firing supervisor can maintain flame lengths sufficient to accomplish the consumption objective.

There must be sufficient variation to work with. The optimal fuel consumption n the test burn would be at midpoint between HEAD and FLANKS of the burn. This flame length becomes the intensity objective.

As an example: For 75% consumption of the live fuel that is coastal sagebrush, the flames need to be 15 to 20 feet. The firing crew will be given this objective flame length to attain. As the burn progresses, alignments will change over time and topography, causing flame lengths to go out of the objective window. If the firing supervisor wishes to change the intensities and consumption the officer will adjust how fire is placed in the fuel bed. Knowing the cause and affects of fuel flammability and the alignment of forces are crucial to the successful management of intensity.

The Firing Supervisor and the Operations Section Chief need to be able to predict the intensity that will occur as fire is placed in various alignments. This test burn data is the basis for predictions of fire in the project burn area under the prevailing conditions.

It is this data gathered by observations of live fire that enable fire officers to give orders and direct the placement of fire on the land.

How many Test Burns are needed?

Each project is different. Test burning is needed to find the range of fire intensity that will occur while in the RX window and at various points on the fuel flammability curve. Burn to find maximum intensity variations that will occur while in the RX window. The burn supervisor must have enough information to decide when to light and how to lay fire on the ground to safely accomplish the objectives of the burn. This is what should decide how many places need to be tested. Testing can continue year around combined with fire drills. The fires are not considered test fires unless the CPS burn form is completed and a video record made of the event.

Creating the Firing Plan

The first step is to obtain a contour map and a photo of the burn area. Label the document as follows:

  1. Compass references
  2. Name of Burn
  3.  Overlay the burn perimeter on the map.
    1. Make the perimeter lines along ridges or canyon bottoms.
    2. Avoid lines with no topographic variations on either side.
  4. Label the Drop Points, the I.C.P., Helispot, Staging areas etc.
  5. Mark and identify Divisions on the map
  6. Mark the predicted wind directions on the map do not overlook local transition wind changes.
  7. Label the sequence and timing for firing the perimeter.
    1. Starting point, direction and time.
    2. Ending point, and time to complete firing.

This last item will determine the number of firing teams required to meet the schedule. The timing of ignitions needs to be done for each aspect. Timing can be expressed as clock hours or the segment of the fuel flammability curve. The test burn will establish the timing limits for each aspect.

Timing is more important in areas where heavier fuels will carry the fire. Where grass is the primary fuel the timing should be based on the information gained during the test burn. Generally speaking soon after grass has sun on it is heated and dried and ready to burn until it is shaded. The timing and sequence plan is for brush types and not as important for grass or fine fuel types.

When burning in brush it is entirely a different story. Brush is greatly affected by the time and aspect as well as the relationship of the heat and flammability of the side of the bush the fire enters. If a burn is done in the morning on a West Aspect, the supervisor needs to realize that the fire behavior intensities will increase until mid afternoon. Since RX burns are usually done during conditions of moderate fire danger, timing the burn near the peak of the fuel flammability curve is generally recommended.

Selecting the fuel flammability range and identifying the time each aspect will be ready to burn determine the timing of the burn. The usual time of peak flammability for an aspect lasts about 4 hours. Ignition timing should be planned by aspect, when the aspect and fuel are near peak flammability. The objective of the timing will be to burn the fuels when they are most flammable. At this time the exposure fuels on the opposite aspect will be cool, a preferred situation for easier control of escapes and spotting. Manage the perimeter lines to that escapes and spotting that may occur will be in areas which are out of alignment and therefore easier to control fire on.

How to Direct Firing Teams

The pre selected fire alignment can be amended as the supervisor wishes by the use of simple key phrases. For more heat, call for “more head fire alignment.” For less intensity, call for “flanking fire or backing fire.” Then it is necessary only to indicate where the string of fire should be introduced to the ground. Remain mentally ahead of the fire by making comparisons of fire intensity and the alignment of forces. If the intensities are within the intensity window, and you make a prediction that there is more force alignment ahead and call for downward fire intensity by placing fire so it will travel in a more out of alignment direction through the fuel.

Copyright © 2010, 2017 by Doug Campbell.

One thought on “Chapter 14: CPS Rx – Fire Modeling


    Since the writing of the CPS RX book on prescribed burning, the Wildfire Management Tools, WMT, has come into being.

    This tool is a revelation when incorporated into the instructions for RX burning.

    The WMT program could add to the development of the forecast fire behavior by adding the wildfire Diamond Symbol to locations on a map of the burn area that shows the fire behavior of the head, flanks and heel.

    The Test burn would have the model calculation as well as the observed ground truth fire behavior, making a comparison to assure the model BEHAVE is going to be accurate.

    The wildfire diamond that depicts the direction of spread and flame lengths can be placed at locations of potential escape from the planned perimeter.

    This calculation would help to determine mitigation needs or assets held in readiness for an escape.

    Firing and timing procedures can be planned using the attributes of the WMT.

    As time goes on, the model can keep up and determine the fire behavior throughout the day or night.

    Thinking of escapes of RX burns, the use of this program would assure that when a burnout was planned, the fire behavior potential would be better assessed. Known flame length of head fire to crown could be foreseen by knowledgeable fire overhead.

    Now we have at our disposal the means to better plan tactics and strategy on wildland fires and RX burn jobs.

    Add to your toolbox.

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