The contrasted response of ash to wetting: The effects of ash type, thickness and rainfall events

- The ash layer has an important role in controlling water infiltration.
- The ash layer has an important role in controlling runoff generation.
- The ash layer contributes to slowing the runoff if is not eroded.
- Rainfall simulations showed that the runoff decreased as ash thickness increased.
- Different processes were observed depending on the ash type.

After a wildfire the soil is covered by ash. Ash properties depend on vegetation type, amount of fuel and fire intensity. The ash layer controls the post-fire soil hydrologic response, but little is known about the effect of ash thickness and ash type on infiltration, which is relevant for post-fire runoff and soil losses and for ecosystems rehabilitation and restoration. This paper analyses the role of i) ash type (black or white), ii) thickness (5, 15 and 30 mm-thick) and iii) temporal variation (0, 15 and 40 days) under three simulated rain events (55 mm for 1 h) on soil surface hydrology. The rainfall was simulated on 0.25 m2 plots, and time to ponding, runoff and runoff discharge were measured. The infiltration rates, the initial infiltration rate (f0), the steady-state infiltration rate (fc), and the infiltration decay factor (k), were calculated and the Horton infiltration equation applied. The results show that soils covered with white ash doubled the runoff rates of soils covered with black ash. In general, runoff decreases as the ash thickness increases and the runoff decreases with the number of rainfall events after the fire in plots covered with white ash. Ponding time and k are positively correlated by the ash thickness and f0 and fc are correlated by the rainfall events (in three runs). Ash type and ash depth are key factors on the soil hydrology after a wildfire.