Can pore-clogging by ash explain post-fire runoff?

Abstract

Ash plays an important role in controlling runoff and erosion processes after wildfire and has frequently been hypothesised to clog soil pores and reduce infiltration. Yet evidence for clogging is incomplete, as research has focussed on identifying the presence of ash in soil; the actual flow processes remain unknown. We conducted laboratory infiltration experiments coupled with microscope observations in pure sands, saturated hydraulic conductivity analysis, and interaction energy calculations, to test whether ash can clog pores (i.e. block pores such that infiltration is hampered and ponding occurs). Although results confirmed previous observations of ash washing into pores, clogging was not observed in the pure sands tested, nor were conditions found for which this does occur. Clogging by means of strong attachment of ash to sand was deemed unlikely given the negative surface charge of the two materials. Ponding due to washing in of ash was also considered improbable given the high saturated conductivity of pure ash and ash–sand mixtures. This first mechanistic step towards analysing ash transport and attachment processes in field soils therefore suggests that pore clogging by ash is unlikely to occur in sands. Discussion is provided on other mechanisms by which ash can affect post-fire hydrology.

Comparing topsoil charcoal, ash, and stone cover effects on the postfire hydrologic and erosive response under laboratory conditions

“…Balfour et al () also concluded that the formation of an ash crust can potentially reduce erosion. In this experiment, it was possible to find an ash crust (see Figure ), and our data showed that it was highly permeable, as pointed out by Stoof et al ().…”

“…This ash‐preferential washing can be attributed to its lower particle size. Char particles contained comparatively more carbon content than white and grey ashes (Balfour, Doer, & Robichaud, ; Merino et al, ; Stoof et al, ), which resulted in a higher particle size, a stronger structure, were not likely to be compacted by raindrop, and, lately, increased surface roughness. Char particles would create more opportunities for runoff impoundment and sediment deposition than ashes (see Figure ).…”

Regional patterns of postwildfire streamflow response in the Western United States: The importance of scale‐specific connectivity

“…Findings demonstrating the unlikeliness of an effect of pore clogging on infiltration in sandy soils (Stoof et al . , ) have shifted the research focus in the Rockies to the effects of differences between hydraulic conductivity of ash and underlying layers depending on texture, delayed water release from ash as a result of capillary forces, and lateral flow through ash and soil on top of a water repellent soil (Baker & Hillel, ; Bodí, Doerr, Cerdà, & Mataix‐Solera, ; Schroth, Istok, & Selker, ).…”

“…, ), especially not in sandy soil because ash and sand both have a negative surface charge (Stoof et al . , ).…”

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

“…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. They discuss other mechanisms by which ash can affect post-fire hydrology, and present results demonstrating the important hydrological effect of ash on saturated hydraulic conductivity.…”