Post-fire rill and gully formation, Schultz Fire 2010, Arizona, USA

Abstract

The Schultz Fire burned 6100ha on the eastern slopes of the San Francisco Peaks 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%. The upper slopes rise to over 3300m and are the source for high energy water, coarse sediments, and woody material. A series of flood events beginning in mid-July 2010 initiated erosion that removed substantial soil from the upper slopes of the watersheds. The second event was characterized by a peak rainfall of 24mm in 10min. Substantial amounts of soil were eroded out of a newly developed rill and gully system, removing the A horizon and much of the B horizon. Prior to the fire there were no rills or gullies as the soil was protected by a thick O horizon. This protective organic layer burned off during the fire leaving the soil exposed to raindrop impact and erosion. There was widespread occurrence of high severity fire, with some watersheds classified as 70% high severity wildfire. This left most of the soils with moderate to severe water repellency. The development of an extensive rill and gully network fundamentally changed the hydrologic response of the upper portions of every catchment. The intense, short duration rainfall of the 2010 monsoon interacted with slope, water repellency and extensive areas of bare soil to produce flood flows an order of magnitude in excess of flows produced by similar pre-fire rainfall events. The rill networks now cover much of the upper slopes and gullies are beginning to cut to bedrock. Sediment delivery to the channel systems is likely to continue unabated for many years and hydrologic response will continue to be flashy.