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Thread: Wet winters may not dampen small wildfires

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    Default Wet winters may not dampen small wildfires



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    Default Re: Wet winters may not dampen small wildfires

    Wow that is interesting small fires essentially behave differently that larger ones depended on the moisture... Who would have though this?
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    Default Re: Wet winters may not dampen small wildfires

    This has been know for a long time. We experience many more light fuel (ie: grass fires) in a year following a wet season, especially so if it is a drier year. You simply have more grasses, forbs etc as they had more moisture available for growth. When they dry out it does not take much to get them going. On the other hand, larger fuels will have had more moisture to thoroughly soak them. More on that below.

    Other than allowing for better growth, Fuel moisture content is a major factor in fire. Simply put, the wetter a fuel is the harder it is to ignite and keep burning.

    Fuel Size and Arrangement also play a role. Light fuels (grasses,needles, twigs etc) take on and release moisture quickly while larger fuels (branches, logs) take much longer to cycle through. A tall grassy field can dry out in a day of hot sun and wind (or get soaked by a light rain) while it can take several days/weeks for larger fuels to dry or take on moisture. If you have ever had a wood stove you know how long it takes freshly cut wood to dry. In wetter years/conditions you can have light fuel fires run through heavier fuels and not involve them. Have a year or two of drought and the same fires will get everything going as more fuel is available. In fact we used this knowledge to do controlled burns and especially under burns. If you time it right you can burn off the surface fuels and not have risk a larger fire or damage to the trees.

    The spatial arrangement means that if you have widely spaced fuel (horizontally and/or vertically) it will be much harder for the fire to spread. On the other hand if you have fuel close to each other the fire can move via convection and/or radiation (conduction not so much). Think of a tall spruce tree with thin branches and needles all the way to the ground. When it is dry they can go up like a candle (hence the term 'candling'). Other trees, like a mature Douglas Fir have few if any lower branches so it takes quite a bit of heat to get the fire up into them. Or, horizontally, think of a dense dry grassy field vs a patchy grassy lot. The first one will burn and spread much easier.

    Another surprise to most people is that fire can burn along and under ground as long as there is dry fuel and enough oxygen getting to it. I had a crew working on a fire which had spread onto an area which had been cleared and flattened. A lot of debris had been buried. It was a few years into a drought and there were enough air gaps around the buried material that they dried out and fire (smouldering) got into them. I had an excavator digging up smouldering material from a good 3 meters down. Same thing is possible with soils with a high organic content. This is why basic training tells you to dig fuel breaks down to mineral soil.

    I could go on for a while but one last thing to know is that not all fire is spread by flame contact, which seems to be the general public's idea of how it works. Fires will lift embers and like paratroopers they will drop ahead (or behind/beside if the wind shifts) of the main fire and create smaller fires if they land in a receptive fuel. These grow and join the main fire. You learn pretty quick that while on a fire you constantly keep looking around and post lookouts if you can't see. Getting entrapped makes a good story for the grandkids but believe me it is not something you want to go through.

    Wildfire management is a lot more than "putting the wet stuff on the hot stuff". There is a lot of science involved and it is a constantly evolving field. I won't go into it much more than I have but with fuel, weather and terrain play a huge role in fire. Just like anything else in the big outdoors it can come around and bite you. At least we can modify the fuels to a degree.

    Some background surfing

    Fire behaviour | Environment and Natural Resources

    Canadian Wildland Fire Information System | Natural Resources Canada - Background Information
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