San Diego Soliloquies

Sunday, November 30, 2003

Hydrophobia, Not Just for Fido

Wildfires in California are post-modernists, imbued with more levels of irony than a Focus on the Family newsletter. The chaparral and coastal sage that dominate our natural ecology are adapted to fire. In order to survive what is naturally a desert (San Diego averages less than 10 inches of rain a year ) the natural vegetation protects its own moisture with oily, waxy resins that are highly flammable. The stuff goes up like a candle. But the genius of evolution means that plants have adapted to this. After a normal fire (usually caused by lightning strikes before Man arrived) the ground is ready for the seeds that the heat of the fire cracked open on the plants. A soft winter rain and we are ready for our 2 weeks of green and flowers, to be followed by 50 weeks of brown scrub.

If the fire is too hot and moves slowly through an area, though, the precise opposite occurs. The resins vaporize in fires greater than 600 C (> 1000 Fahrenheit). Because soil is an excellent insulator, when the vapors get to the soil and sink in, they solidify a few centimeters below the surface, forming a hydrophobic barrier. When the heavy winter rains arrive, they are stopped by the barrier, and saturate the soil above. If the soil is on any kind of slope, you get a mudslide. We call this the Fire and Flood Cycle. Just when you're about to start rebuilding, your house may take a ride down the hill, or your neighbor's house may arrive uninvited. Expect these visuals on the national news some time in January

If you keep natural fires from the chaparral the fuel load builds up, and the more fuel you have, the hotter the fire. I have no grand answer for this, no policy statement to make. Maybe we can find a way to do preemptive burning in the areas that will regrow. For the areas that did not burn we will just have to stand guard.


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