EVALUATING FIVE YEARS OF SOIL HYDROLOGIC RESPONSE FOLLOWING THE 2009 LOCKHEED FIRE IN THE COASTAL SANTA CRUZ MOUNTAINS, CALIFORNIA

The Lockheed Fire burned 31 km² of the Scotts Creek Watershed in Santa Cruz County, California in August 2009. Swanton Pacific Ranch, a research facility of California Polytechnic State University, San Luis Obispo, is located within the burned region. This extensive wildfire presented a unique opportunity for studying the hydrologic response of burned soils in the Santa Cruz Mountains where there is insufficient post-fire data and a lack of understanding of how fire effects soil hydraulic properties. Soil infiltration and hydrophobicity characteristics were evaluated with rainfall simulations, Mini-disk Infiltrometer and water drop penetration time tests at sites across a range of burn severities, soils, and vegetation types throughout the Scotts Creek watershed each year for five years following the burn. The three data sets were analyzed using mixed effects modeling to evaluate effects on infiltration rates and water repellency.

Data from the 54 rainfall simulations reveal that time since the burn was a highly significant predictor (p=0.005) of infiltration, but that burn severity is not a significant predictor of infiltration and runoff rates. Although there was no significant difference in the change of infiltration rate and runoff between burn severity groups together, comparing high to all other burn severity groups reveals that burn severity (p=0.023) was a statistically significant predictor for runoff. This can be attributed to the physical nature of high burn severity sites including shallow soils, fire induced soil water repellency, and little to no duff layer.

Data from Mini-disk Infiltrometer tests conducted at 1 and 3 cm depths reveals that time, depth, burn severity, and the interaction between burn severity and time had a significant impact on infiltration volumes. Data from water drop tests conducted at 1, 2, and 3 cm reveal that depth, burn severity and the interaction between burn severity and time did not significantly affect infiltration times.

While some results showed that fire had a significant impact on the hydrologic response over time, a key part to understanding this impact lies within the physical characteristics of the sites including varying vegetation communities, soils, and geology of the Coastal Mountains which, independent of fire, highly influence infiltration and runoff.