Evolution of overland flow after a severe forest fire, Point Reyes, California

Forest fires on granitic soils often increase overland flow and erosion. Runoff generation was monitored on a small hillslope plot on Mt. Vision near Point Reyes Peninsula, California, after it had been burned by a wildfire on October 3, 1995. After the fire, the ground surface was covered with up to 2 cm of ash, which overlaid a 5–20 cm thick hydrophobic (water repellent) soil layer. We used nine recording tensiometers to monitor soil-water potentials during infiltration and runoff. Surface-runoff rates were determined by diverting the flow into a collection tank. The subsurface flow through the upper 6 cm of soil was collected and measured in a second tank. The surface runoff was diverted to a tank in order to record its rate. The initial intense rainfall infiltrated into the base of the ash-bed; here, the hydrophobicity limited deeper penetration and led to both subsurface and shallow saturation overland flow. The preferential flow paths through the ash layer contributed to deeper water penetration. As the ash was eroded and consolidated with successive rainstorms, the preferential flow paths clogged, the infiltration capacity reduced, thus preventing the storage of shallow permeable soil; therefore, the runoff generation changed to Hortonian overland flow. Correspondingly, the runoff ratio increased from approximately 0.2 during the early storms to 0.8 during intense rain bursts. These results suggest that runoff mechanisms evolve simultaneously with the eroding soil surface.