EFFECTS OF FOREST FIRE ON SOIL WETTABILITY; FRENCH FIRE RECOVERY AREA, NORTH FORK CALIFORNIA

Laboratory and field experiments were carried out to determine the effects of forest fires on soil water repellency or hydrophobicity in areas burned by forest fires in the Sierra Nevada Mountains. It was noticed in the field the surface soil was ashed and water-wettable, but just two inches below the surface the soil was dry and hydrophobic. Soil samples from the surface and two inches below were collected from several locations within the boundaries of the French Fire recovery area in North Fork, CA, representing differing severities of fire damage ( high burn and moderate burn). Scientific Water Drop Penetration Time (WDPT) tests were conducted on each of the soil samples collected. Results show that the WDPT ranged from 0.2 seconds in moderate burn areas to more than 60 minutes in high burn areas. Water ponding tests were then performed on all soils of severe repellency (WDPT > 30 seconds) to determine the water-entry or hydrostatic pressure needed for water to start infiltrating. The water-entry pressure of the severely burned soils reached 6.5 to 7.0 cm of water head. Mini-Disk infiltrometers were also used in the laboratory to determine the rate of infiltration of water and ethanol in the soils. We found that in the high burn areas there was no infiltration of water into the soil due to severe water repellency. However, ethanol was able to infiltrate into the hydrophobic soil (two inches below the surface). Thus the ability of ethanol infiltration is an indication of soil hydrophobicity. The results indicated that in areas of moderate burn severity, the surface soil (top two inches) was more water repellent, whereas in areas of high burn severity, the subsurface soil (second two inches) was more water repellent.