I first heard the term on the local evening news in fall of 2018: Meteorologist Kris Kuyper was talking about hydrophobic soils. This potentially catastrophic natural phenomenon seems counterintuitive: soils which are damaged by the intense heat of fire become water repellent. Their post-fire inability to absorb and filter rainfall can cause problems with erosion and runoff.
Adding insult to injury, fire also burns plant roots that can help stabilize the soil, and destroys plant stems and leaves that slow rainfall’s contact with the ground surface, allowing more time for percolation into the soil. The chemistry of soil hydrophobia is fairly basic. Plant materials that burn hot release a waxy substance that penetrates the soil while still in gas form.
It takes very high temperatures to produce this gas which coats soil particles when it cools and becomes solid. To the naked eye, hydrophobic soils look like their non-hydrophobic counterparts. But biologically speaking, they have now become latent disaster zones.
According to an article by Douglas Kent (“First Aid for Sonoma County’s Fire-Damaged Soil,” Santa Rosa Press Democrat, October 27, 2017), “erosion leaps as high as 200 percent following fires in urbanized areas.” The slopes and vales that comprise the mountain and foothill communities of our region mean that the immediate danger from water repellent soil comes in the form of flash flooding and the flow of debris and mud. Hydrophobia in burn-scarred soils mak.