By Wendy Langhans
Earlier this week, while making a right turn on to McBean Parkway, the unexpected happened – the rear end of my car hydroplaned. Just a bit. Not unmanageable. But a wake-up call nonetheless. You see, the rain, combined with the oil and grease that had accumulated over the dry summer months, created a thin, slippery film that “pulled the rug out” from under my rear tires.
But roads aren’t the only surfaces with a waxy coating right now – so is the land burned in our recent wildfires. How did this waxy coating form and what does it mean, especially now as we enter the rainy season?
Wilson Canyon, after the Sayer fire in 2008.
Fire scientists refer to this waxy coating as “hydrophobic soil“. It can be found 1/2 to 3 inches below the surface of the soil. These soils are formed in a two-step process. (1) A wildfire vaporizes the aromatic oils, resins and waxes found in plants, especially Chaparral plants. (2) The vapors penetrate the soil and eventually solidify as they reach the cooler temperatures below the surface, creating a waxy coating around the soil particles. Hydrophobic soil can be 1 to several inches thick, depending on the severity of the fire (higher temperatures means deeper penetration) and the type of soil (larger pores allow more vapor to penetrate).
From the Smithsonian in Washington, D.C. – a guide to the layers in soil.
Hydrophobic means, “to repel water”, which is exactly what this waxy-coated layer of soil does. When the rainy season arrives, the hydrophobic soil reduces the amount of water than can infiltrate the soil, leading to an increase in both runoff and soil erosion.
Eventually, soil microorganisms and plant growth will break down this hydrophobic layer, but the process can easily take more than a year. In the meantime, soil will continue to slip-slide away during the winter rains.
Upcoming Outdoor Events:
Saturday, October 17, 8-10 AM. Bird walk in Towsley Canyon. All year round, the habitats of Towsley Canyon attract a wealth of birdlife. Beginners are welcome. Bring your binoculars. 2 hours, easy walk. For a map, go here.
You can listen to stories like this every Friday morning at 7:10 a.m. on “The Hike Report”, brought to you by your hometown radio station KHTS (AM1220) and by the Mountains Recreation and Conservation Authority.