adj. Resistant to penetration by water; not wettable. (n. Water repellency)
The term water repellent is usually used to describe an object, fabric, or a substance that repels water because it contains or has been coated with a hydrophobic compound. In the case of fire-induced soil water repellency, it is the latter: soil particles are coated with hydrophobic compounds. When organic material burns at high intensity, the hydrophobic organic compounds often vaporize, and some of the vaporized compounds move down into the soil. When the vapors reach a soil depth where the temperature is low enough, the hydrophobic compounds condense and coat the soil particles at that depth, generally 0.25 to 2.0 inches (0.5 to 5 cm) below the surface.
This near-surface water-repellent soil layer inhibits infiltration and contributes to increases in runoff and erosion after fires. Dry, strongly water-repellent soil resists penetration by water; when the water repellent soil is exposed to moisture for extended periods of time (for example, during snowmelt) the soil water repellency weakens and may temporarily disappear. When the soil dries, water repellent conditions reoccur. Over time, as the soil is intermittently exposed to moisture, fire-induced soil water repellency slowly declines.