Measuring Forest Moisture Content With the U.S. Forest Service

I recently went out with a couple of Forest Service firefighters to learn how they calculate the fire threat across Colorado. My interest in the Forest Service’s efforts to gauge wildfire threat levels started this winter when I was at a SNOTEL site measuring the snowpack atop Berthoud Pass. My co-worker and fellow meteorologist Cory Reppenhagen was with me when we saw the far greater than average snowpack and made the assumption that the fire threat will be much lower as a result… which was an erroneous assumption to make. As Cory and I left the SNOTEL site I thought, “I wonder how they know how much water actually gets into the trees. Lots of snow doesn’t mean that the water is actually getting into the trees, does it?”

As it turns out, the season snowpack can help keep the wildfire threat low, but only so long as there is snowpack. As soon as the snowpack melts, the trees rapidly dry out. Coming out of dormancy and going through the summer, all fire fuels (like trees, grass, and ground cover) need a steady supply of moisture to maintain that low fire threat. I should point out, a good snowpack can lead to quick growth of the entire vegetation, not just the trees, so consistent rain is needed to keep that vegetation from turning into fuel.

The Forest Service measures moisture content in these fuel sources, and does so regularly. Cory and I went along with a crew in the Pike National Forest in early August to see what it was all about.

We hit their Trout Creek fuel site, one of many spread throughout the forest. Samples were taken of the ground cover (litter), the ground layer just beneath a tree canopy (duff), living pine needles, and 3 various sizes of a tree (large twigs, medium to large branches/trunk). The collected samples are then “baked”.

This process meant adding all the collected fuel samples to a convection oven and baking them at 170 degrees for about 24 hours. The weight of the fuel sample is then compared to its weight before going into the oven. The difference in weight is the moisture (water) loss during the dehydration, which indicates the fuel’s moisture level.

These samples are collected at dozens of sites in Colorado, and hundreds across the Western US every two weeks. The compilation of the sites produces quick outlook images such as the one below, showing the moisture levels in the small to medium size branches (the 100-hour fuel as the Forest Service calls them). This image is from early August when Colorado’s steady supply of rainfall had high moisture content in this fuel source, and all other fuels measured quite wet as well.

I asked the Forest Service how long a period of dry weather it would take to see the fuels trend drier, increasing our fire threat. Their response: only a week!

I had been waiting for a good period of drier days to see the fuel moistures change. This past week, and especially weekend, was just what I needed. Statewide temperatures were near to above average, and rainfall was near to below average. See the comparison in the fuel moistures to two weeks ago.

100-Hour Fuels (the small to medium branch sizes):

Notice the drying that occured in both fuels, in a time period of two weeks. It makes sense that the smaller fuel dried in a more pronounced way, but even the large fuels showed drying across Colorado.

It is the combination of these fuels that make up our fire threat. These fuel samples help determine staffing for the Forest Service as well. On the day of our visit, several firefighters from that station were already working the fires in the Pacific Northwest where the threat was clearly greater. Yes, the fire threat in Colorado remains low, but that can change more quickly than I had previously thought.

As my colleagues and fellow Weather5280 bloggers have pointed out, the moisture and temperature outlook will change very little over the next few weeks. However, if continued drying occurs, these fuels will be more prime to ignite. Most of Colorado’s fires are started by lightning, though careless campfires are also a threat.

For the next week, the most likely region for an elevated fire threat (including factors of dry thunderstorms, wind, and heat) will be in Southern Wyoming to Northwestern Colorado.

My thanks to Lawrence Lujan, Jay Karle, and Samuel Urffer with the Forest Service.

Matt Makens

Matt Makens has won 5 Emmys for his weather coverage. He has the seal of approval from the NWA and is a certified Broadcast Meteorologist as designated by the AMS. He works for Colorado's Own Ch 2.

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