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FOREST, RANGE & WILDLAND SOILS

Causes of Post-Fire Runoff and Erosion: Water Repellency, Cover, or Soil Sealing?

^a Dep. of Forest, Rangeland, and Watershed Stewardship, Colorado State Univ., Fort Collins, CO 80523
^b Golder Associates Inc., 18300 NE Union Hill Rd., Suite 200, Redmond, WA 98052
^c Dep. of Agronomy, Purdue Univ., West Lafayette, IN 47907
^d INIFAP, Centro Regional de Investigación del Pacifico Centro, Parque Los Colomos s/n, Guadalajara, Jalisco 44660, Mexico

^* Corresponding author (larseni{at}u.washington.edu).

Few studies have attempted to isolate the various factors that may cause the observed increases in peak flows and erosion after high-severity wildfires. This study evaluated the effects of burning by: (i) comparing soil water repellency, surface cover, and sediment yields from severely burned hillslopes, unburned hillslopes, and hillslopes where the surface cover was removed by raking; and (ii) conducting rainfall simulations to compare runoff, erosion, and surface sealing from two soils with varying ash cover. The fire-enhanced soil water repellency was only stronger on the burned hillslopes than the unburned hillslopes in the first summer after burning. For the first 5 yr after burning, the mean sediment yield from the burned hillslopes was 32 Mg ha^–1, whereas the unburned hillslopes generated almost no sediment. Sediment yields from the raked and burned hillslopes were indistinguishable when they had comparable surface cover, rainfall erosivity, and soil water repellency values. The rainfall simulations on ash-covered plots generated only 21 to 49% as much runoff and 42 to 67% as much sediment as the plots with no ash cover. Soil thin sections showed that the bare plots rapidly developed a structural soil seal. Successive simulations quickly eroded the ash cover and increased runoff and sediment yields to the levels observed from the bare plots. The results indicate that: (i) post-fire sediment yields were primarily due to the loss of surface cover rather than fire-enhanced soil water repellency; (ii) surface cover is important because it inhibits soil sealing; and (iii) ash temporarily prevents soil sealing and reduces post-fire runoff and sediment yields.

Abbreviations: CEC, cation exchange capacity • CST, critical surface tension