Extensive Erosion In High Altitude Alfisols After the Schultz Fire 2010, Arizona.

Abstract: The Schultz Fire burned 6,100 ha on the eastern slopes of the San Francisco Peaks, a dormant Holocene to Middle Pliocene stratovolcano in northern Arizona. The fire burned between June 20th and 30th, 2010, across moderate to very steep ponderosa pine and mixed conifer watersheds. About 40% of the fire area was classified as high-severity, mostly on mountain slopes greater than 30% and in places exceeding 100%. A series of flood events beginning in mid-July 2010 initiated erosion that eroded soil from the upper slopes of the watersheds. The second event of a storm series had a peak rainfall intensity of 24 mm in 10 minutes. Substantial amounts of soil were eroded out of a newly developed rill and gully system, removing the A horizon and, in places, much of the B horizon. Prior to the fire, there were no rills or gullies in the Mollic Eutroboralf and Eutric Glossoboralf soils as the surface was protected by a thick (10 to 30 cm) O horizon. This organic layer was burned off by high severity fire leaving the mineral soil exposed to raindrop impact and erosion. The development of an extensive rill and gully network fundamentally changed the hydrologic response of the upper portions of every catchment. Mineral soil loss is >10 cm on 2,000 ha. Rills are now 10-20 cm deep running the entire length of slopes. Small gullies (50–100 cm deep) merge into now deeply incised drainage channels (3–5 m deep). Intense, short duration rainfalls of the 2010 Monsoon interacted with slope, water repellency and bare soil to produce erosion and flood flows 1-2 orders of magnitude in excess of flows produced by similar pre-fire rainfall events. Because of the steep slopes and high-intensity rainfall, prevention of the post-fire erosion was impossible.