Event-scale erosion of scoria cone Hillslopes in the western San Francisco Volcanic Field, Arizona
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A field study was conducted to collect event-scale erosion data in the San Francisco Volcanic Field, Arizona, from 1996 to 2000. This Neogene scoria cone field consists of late Miocene to Holocene volcanic cones, at various stages of degradation. The study area has a semiarid climate with much of the rainfall occurring in July and August, which was convenient for the collection of field data. Being the first study of its kind to use an integrated set of degradation data for natural hillslopes, collected specifically for this purpose, the results of the study can be used to validate and improve existing hillslope erosion models. Data on linear diffusion-type degradation by rainsplash, and on non-linear type degradation by overland flow were collected with the help of splashboards, erosion pins, rain gauges, global positioning system, and cross-sections. Degradation is defined as a change in surface height and the measured rates of degradation are compared to long-term degradation models based on mass-conservation equations. Effective diffusion coefficients, erosion rates due to raindrop detachment and overland flow estimated empirically are investigated with this data set. The rainsplash data are consistent with a long-term diffusion model of slope degradation. However, the data are consistent with an arid climate than with the semiarid climate of the San Francisco Volcanic Field. This indicates that previous diffusion-type modeling of scoria cones may have overestimated the contribution of quantity of rainsplash erosion and underestimated the contribution of overlandflow processes to overall erosion. The short-term slope modification by overland flow compares well with results from a cellulax automaton, smooth particle hydrodynamics model. Detailed measurements of horizontal and longitudinal profiles on the slopes give in-depth insight to the processes shaping these landforms. A severe forest fire damaged one of the two hillslope observation sites in 1996. This event gave us the opportunity to observe the impact of wildland fire on erosion. The rate of degradation of the burned scoria cone was of orders of magnitude larger than that of the unburned cone. Both diffusive and non-diffusive erosion models predict that, the first post-fire rainy season in areas affected by fires, are severe.