Effectiveness of soil erosion barriers to reduce sediment connectivity at small basin scale in a fire-affected forest

[Abstract:] Forest fires and post-fire management practices (PFMP) cause changes in the hydrological response of a hillslope. This study evaluates the effect of log erosion barriers (LB) and Easy-Barriers® (EB) on the spatial patterns and values of structural sediment connectivity (SC) in a Mediterranean mountainous pine forest affected by an arson fire in August 2017. A drone flight was done in July 2019 (23 months after the fire and 11 months after the PFMP) to obtain a high-resolution orthomosaic and DEM (at 0.05 m). Two contrasted areas, with and without PFMP, were selected along the same hillslope and 26 small basins were identified: 16 in the treated area (mean area, slope and vegetation recovery of 916 m2, 60% and 25%; with 94 LB and 39 EB) and 10 in the untreated area (1952 m2, 75% and 20%). The aggregated index of sediment connectivity (AIC) was chosen to compute SC in three temporal scenarios: Before and just after the fire and when all PFMP were implemented including the incipient vegetation recovery. Output normalization allowed the comparison of the non-nested basins among them. After accounting the intrinsic differences among the basins and areas, and the temporal changes of SC between the three scenarios, the contribution of the barriers was estimated in 27% from the total decrease of SC in the treated area (−8.5%). The remaining 73% was explained by the vegetation recovery. The effectiveness of the LB (11.3% on average) and EB (13.4%) did not diminish with increasing slope gradients. These percentages become relevant considering the small area affected by the LB (2.8%) and EB (1.3%). Independent metrics (convergence index, flow width, flat areas and LS factor) also reported clear differences between the two areas –higher soil erosive intensity in the untreated area– and in accordance with the AIC results.

This research was funded by the project SPECTORS, which is a Dutch-German cooperation project funded by INTERREG V-A Deutschland-Nederland. This research was also included in the research activities of the European COST Action FIRElinks (CA18135) “Fire in the Earth System: Science & Society” (European Union Framework Programme Horizon 2020). Dr. Manuel López-Vicente, as researcher involved in the FIRElinks COST Action, was awarded with a Short-Term Scientific Mission (STSM) grant to do a short-stay at Wageningen Environmental Research with Dr. Saskia Keesstra. We thank Mr. Agustín Bermejo Fernández, from the company ‘Agroforestal Monte Vivo S.L.’, for giving us access to the research area, information about the wildfire event and technical information and pictures of the ‘Easy-Barriers' used in the study area.