Wettability of ash conditions splash erosion and runoff rates in the post-fire

Abstract

Although the impact of ash on the hydrological and erosive response of burned soils has been studied in the last years, the effect of ash wettability on particle detachment by splash erosion has been rarely studied. In this research, we have studied the effect of wettable and water-repellent ash on the intensity of splash erosion after a prescribed burn in a Mediterranean shrubland. Runoff rates from wettable and water-repellent ash layers were also studied by rainfall simulation experiments (6mmmin−1 during 10min) at plot scale. Splash erosion experiments showed that the intensity of ash water repellency strongly conditioned the detachment and mobilization of ash and soil particles due to raindrop impacts in the post-fire. Although the intensity of water repellency and the amount of ash decreased through the experimental period, it was observed that splash erosion in originally water-repellent ash areas was about three times higher than in wettable ash areas. Plot-scale rainfall simulation experiments showed that runoff rates were much higher in water-repellent ash areas, where runoff rates increase with decreasing depth of the ash layer. In water-repellent ash areas, thick ash layers store a significant proportion of water that does not infiltrate soil, but contributes to relatively low runoff rates. In contrast, thin water-repellent ash layers are rapidly redistributed by rainfall, and the burned mineral soil surface is exposed. Pore clogging with fine water-repellent ash particles enhances runoff rates. During low or moderate intensity storms, patches of wettable/water-repellent ash arrange in a pattern that produces water infiltration/runoff generation areas, reducing the connectivity of water and sediments through the slope.