What Causes Runoff and Sediment Yields to Increase After Wildfires?

Runoff and sediment yields can increase by several orders or magnitude after high severity wildfires. These increases have been attributed to soil water repellency, loss of surface cover, and soil sealing by either mineral or ash particles, but the relative effects of these factors have rarely been isolated. The objectives of this study were hillslopes burned in high-severity wildfires, 13-34 unburned hillslopes, and 3 hillslopes where the surface cover was removed by raking; and 2) use rainfall simulations to determine whether surface sealing is more prevalent on bare soils or soils covered with varying amounts of ash. The field measurements were made over a five-year period in ponderosa pine forests in the Colorado Front Range. The burned hillslopes generally had stronger soil water repellency than the unburned hillslopes only for the first summer after burning, but the mean cumulative sediment yield from the burned hillslopes was 31 Mg ha-1 as compared to minimal sediment yields from the unburned hillslopes. The raked hillslopes had very similar sediment yields to the burned hillslopes when they had comparable surface cover, rainfall erosivity, and soil water repellency. The rainfall simulations on bare soil generated much more runoff and sediment than the simulations on ash-covered soil, and both bare soils developed a thin, structural soil seal. Runoff and sediment yields decreased as ash thickness increased, but successive simulations quickly eroded the ash cover and increased runoff rates to the levels observed for bare soil. The results indicate that: 1) post-fire sediment yields are primarily due to the loss of percent cover rather than fire-enhanced soil water repellency; 2) surface cover is important because it controls the extent of soil sealing; and 3) ash temporarily prevents soil sealing and reduces post-fire runoff and sediment yields. The results have important implications for forest management and mitigating post-fire erosion.