Assessing the Role of Ash in Rainfall Runoff Dynamics in Fire Affected Soils in Vall D'albaida. Part 1. Field Rainfall Simulation Experiments.

Forest fires used to take place during Mediterranean summer droughts from June to September. Autumn is the wettest season in Eastern Spain and the high intensity rainfall events take place in this period of the year (i.e. > 800 mm in two days in Gandia in 1987; > 600 mm in two days the Massís del Caroig in 1982). Bare soils after a fire event are very vulnerable to erosion due to heat impact changing the soil properties, the lack of vegetation and water repellent layers that develop due to condensation of hydrophobic compounds along the soil profile. However, ash is present and the scientific community is still discussing about the effect of the ash as the thin ash particles can clog the pores, but also the ash can absorb large quantities of water and reduce overland flow, and increase infiltration as it acts as a mulch, which protects the soil. Simulated rainfall experiments (0.25 m2) were carried out at 55 mm h-1 in October of 2010 (very dry soils: < 12 % soil moisture, before autumn rains) during one hour on sixty 1 m2 plots. Runoff was collected every minute and from this information the runoff coefficient and infiltration rate was calculated. Every 5 minutes, runoff samples were desiccated and the sediment measured. From the runoff discharge and the sediment concentration the soil erosion rates were calculated. The results show that the ash cover and ash depth control the rainfall runoff dynamics. GL2008-02879/BTE, LEDDRA 243857 and RECARE (nº 603498, http://recare-project.eu/), iSQAPER (H2020-SFS-2014-635750-2) and CGL2013-47862-C2 supported this research.