Water repellency by laboratory burning of four northern Rocky Mountain forest soils

Abstract

Highly variable water repellent soil conditions have been reported after forest fires. We examined interactions among heating, soil water content and soil texture on water repellency. Undisturbed, 305 mm diameter cores were collected in the field from four soils commonly referred to as ash-cap, mixed ash-cap, no ash-cap and granitic soils. Three artificial burning treatments and a control (no heat) and two soil water contents were evaluated under laboratory conditions. Twenty water drops were placed on each soil layer starting at the surface and continuing at 10 mm intervals to a depth of 70 mm; and the times to infiltrate were recorded. The dry control treatment was more water repellent than the wet control treatment. The dry, low heat treatment was the most repellent, 10–20 mm below the soil surface with mean water drop penetration times greater than 60 s. Repellency decreased as the heating increased. In wet soils of the high heat treatment, a water repellent layer was generally detected 30–50 mm below the soil surface. Presumably, hydrophobic substances were translocated along the temperature gradient which cooled at depth (<50 mm) causing condensation on the soil particles. Water repellency after prescribed fire would probably be minimal because long heating times are not common.