OBSERVATIONS OF HILLSLOPE DEGRADATION ON SCORIA CONES IN THE SAN FRANCISCO VOLCANIC FIELD, AZ, FOLLOWING THE HOCHDERFFER FIRE, 1996

A unique field study was conducted to collect event-scale erosion data in the San Francisco Volcanic Field, AZ, from 1996 to 2000. This Neogene scoria cone field consists of late Miocene to Holocene volcanic cones, with a wide range of degradation stages. The semiarid climate makes the area exceptionally suitable for studies of extreme erosion events, because of the annual intense July and August monsoon season. Being the first study to collect an integrated set of natural hillslope degradation data, the results can be used to validate and improve existing computational models. Data on rainfall, rainsplash, overland flow and soil creep were collected with the help of splashboards, erosion pins, erosion plots, raingauges and channel cross-sections on slopes of 0 to ~38 degrees. Rainsplash data are consistent with a long-term diffusion model of slope degradation, but measured sediment fluxes are slightly higher than those predicted by such a model. Data on degradation by overland flow are not consistent with any type of diffusive-degradation model.

A severe forest fire damaged one of two hillslope observation sites in 1996. This has given us the opportunity to observe the impact of fire on erosion due to drizzle (<3 mm*h-1) and to heavy rainfall (>15 mm*h-1), and to different severities of fire damage. The degradation of the affected cone has been accelerated in areas of high fire severity by almost one order of magnitude. In particular, the rate of degradation by overland and pipe flow increased.