Abstract:
Soil infiltration and wettability measurements during the first year following a broadcast burn in the Siskiyou Mountains of
southwest Oregon, have illustrated the magnitude of the effects of light-to-moderate intensity burning on hydrological soil
properties. A prescribed fire near White Creek in late spring significantly reduced soil infiltration for 4 months following the
burn, but infiltration rates below the maximum 100 year precipitation event for the area were not observed. The lowest infiltration rate recorded on logged and burned soil was 5.3 cm/hr (2.1 in/hr), but 94% of all the observations ranged between 9.0 and 11.4 cm/hr (3.5 and 4.5 in/hr). Infiltration rates recorded on logged and unburned soil were greater than 11.2 cm/hr.
Broadcast burning caused hydrophobic substances in the litter and duff layers to become volatilized, subsequent condensation of these substances on soil particles located in the 0-5cm depth of soil. A total of 25% of the exposed mineral soil surface in the
burned section was water repellent 9 days after burning, but this was reduced to 6% within five months. Some naturally-occurring water repellency existed in the unburned condition, yet the greatest percentage recorded was only 1% in August, when soils were at their driest. In assessing the wetting difficulties of a soil sample, the
measurement of an apparent liquid-solid contact angle was more consistent than obtaining water drop penetration times. The
penetration time of a water drop is dependent on the wetting difficulties and pore geometry of the soil directly beneath it, therefore measurements were highly variable for any one soil sample. Regression models correlated infiltration rates with soil contact angles in the burned section. Association was strongest (r² = .93) for infiltration rates obtained on unsaturated soils in which
attraction forces between the soil particles and water molecules predominate. Since this attraction is inversely related to the
liquid-solid contact angle, infiltration rates decreased with increases in the liquid-solid contact angle. During the summer, residual vegetation in the unburned section significantly reduced percent soil moisture below levels recorded in the burned condition. Increased precipitation and lower
evapotranspiration demands, combined with rapid growth of resprouting and invading vegetation in the burned section during early fall, probably led to soil moisture becoming nearly equal in both conditions by mid-fall.
Supplemental hydrological soil measurements, collected 33 days after a moderate intensity broadcast burn at Shan Creek, did not
markedly differ from those obtained 44 days after the light-to-moderate intensity burn at White creek. Although Shan Creek did have
a greater percentage of water repellent soil (52% vs. 12%), there was not a significant difference between infiltration rates obtained at both sites.
