During an intense fire, these substances move into the mineral soil. Some soil fungi also excrete substances that make the litter and immediate surface layer repel water. Fire-induced water repellency can affect soil and the watershed. Hydrophobic soils repel water, reducing the amount of water infiltration. Decreased infiltration into the soil results in damaging flows in stream channels.
Erosion increases with greater amounts of runoff, and much of the fertile topsoil layer is lost.
Increased runoff carries large amounts of sediment that can spread over lower lying areas, clog stream channels, and lower water quality. Depending on the intensity of the fire, hydrophobic layers can persist for a number of years, especially if they are relatively thick. A smaller amount of water will penetrate the soil and be available for plant growth. Not all wildfires create a water-repellent layer. Four factors commonly influence the formation of this layer.