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

Abstract

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.

Identification of runoff generation processes using hydrometric and tracer methods in a meso-scale catchment in Rwanda

“…The range of runoff coefficients found in this current study (16.7-44.5 %) agrees with the range for agricultural dominated catchments found, e.g. by Larsen et al (2007). This gives a hint towards the importance of infiltration and subsurface flow generation during events.…”

Synthesising empirical results to improve predictions of post-wildfire runoff and erosion response

“…Despite doubts (Gabet and Sternberg 2008) and a lack of experimental data, reduced infiltration and increased overland flow due to clogging of pores by ash has been a generally accepted effect (e.g. Larsen et al 2009). Stoof et al (2016) use laboratory experiments to explore whether this process does occur, but find that, at least for the sand used in their experiments, the presence of ash in the pores would be unlikely to promote enhanced overland flow.…”

How wildfires affect soil properties. A brief review

“…The effects of ash on postfire runoff and erosion depend [i] on its physical and mineralogical properties: particle size, porosity, calcium carbonate content, or water repellency (Larsen et al, 2009;Woods and Balfour, 2010;Bodí et al, 2012b), [ii] on physicochemical changes in ash after the interaction with the atmosphere and water (Etiegni and Campbell, 1991), the thickness of the ash layer , and [iii] lithology and soil type of the area affected by the fire (Bodí et al, 2012b;Larsen et al, 2009;Woods and Balfour, 2010).…”

“…The study of the thickness of the ash layer shows the degree of soil protection in the period immediately after the fire, and how it changes in space and time (Pereira et al, 2013c). Several studies have been conducted on the effects of ash on soil properties in burned areas which consider the thickness of the ash layer as a key to understand the impact on soil fertility on the evolution of ecosystems in the post-fire (Mallik et al, 1984;Leighton-Boyce et al, 2007;Cerdà and Doerr, 2008;Gabet and Sternberg, 2008;Onda et al, 2008;Balfour, 2008, 2010;Larsen et al, 2009;Zavala et al, 2009;Pereira et al, 2013bPereira et al, , 2013c. The spatial variability of the thickness of the ash layer may be affected by factors such as soil properties and ash texture.…”