Exploring the effect of hydrological connectivity and soil burn severity on sediment yield after wildfire and mulching

Soil erosion can potentially threaten different resources inside and outside of burned areas, and the risk of water becoming contaminated with sediment may be particularly severe. Various postfire actions, such as applying straw mulch, have been carried out in northwestern Spain in recent years with the aim of mitigating the risk of soil erosion. Nonetheless, because of the short interval between summer wildfire and autumn rains, careful selection and prioritization of the areas to be treated is crucial. Changes in hydrological connectivity could be measured and used as a criterion for selecting such areas. However, studies addressing changes in hydrological connectivity as a consequence of forest fires are scarce. In the present study, we assessed the effects of fire and postfire helimulching on the hydrological connectivity and sediment loads in a forest catchment burned by a wildfire in August 2016. Sediment yields were recorded in 20 plots (180 m²). Hydrological connectivity was computed with a version of the Borselli index and two alternative weighting factors: the C factor from the Revised Universal Soil Loss Equation and a factor based on field surveys of soil burn severity. The effect of mulching was also considered. The results indicate that the version of the Borselli index based on field measurements of soil burn severity best reflects the susceptibility to postfire sediment delivery. Moreover, this method was also suitable for evaluating the effect of mulching on soil erosion. The study findings may help forest managers to plan postfire actions.