The effects of soil texture and ash thickness on the post-fire hydrological response from ash-covered soils

Ash increases the post-fire hydrological response to rainfall in some situations but decreases it in others, and the reasons for this variability are not well defined. We used simulated rainfall experiments to determine whether the varying hydrological effect of ash can be attributed to differences in the underlying soil texture or the ash thickness. We compared the infiltration rates: (1) before and after controlled burns in 0.5 m2 plots underlain by two sharply contrasting soil textures (sandy loam and gravelly silt loam), and; (2) before and after the addition of 0.5, 2.5 and 5.0 cm of ash to the burned sandy loam plots with the original ash layer removed. The controlled burns left a ∼1 cm ash layer comprised mostly of silt and fine sand particles that clogged the largest pores in the sandy loam soil, reducing the final infiltration rate from 91 mm h−1 to 35 mm h−1, but had no effect on infiltration in the silt loam plots. Pore clogging also reduced the final infiltration rate by 20 mm h−1 (40%) and the total infiltration by 16 mm (24%) when 0.5 cm of ash was added to the sandy loam plots. However, increased storage in the ash layer combined with slight increases in the final infiltration rate (by 5 and 18 mm h−1, respectively), increased the total infiltration by 16 mm (30%) and 18 mm (26%), respectively with the thicker ash addition treatments. Thus, the varying effect of ash on infiltration and runoff can be at least partially attributed to between-site differences in soil texture and ash thickness. A thin ash layer (<1 cm) overlying a coarse or macroporous soil will clog the larger pores, increasing the hydrological response, whereas the same ash overlying a fine or non-macroporous soil will have no effect. With thicker ash layers (2–5 cm) storage effects increasingly delay and reduce the runoff response to the point where no overland flow is produced regardless of any pore clogging effect in the underlying soil.