Forest fire effects on sediment connectivity in headwater sub-catchments: Evaluation of indices performance

Forest fires and post-fire practices influence the hydrological response of the soil in terms of runoff and sediment connectivity (SC). In this study, the ability of four indices (IC-Borselli, IC-Cavalli, IC-Persichillo and aggregated index of connectivity (AIC)) to assess SC was evaluated in three Mediterranean headwater sub-catchments (66, 143 and 194 ha) affected by an arson fire in 2012. Three temporal scenarios (before the fire, one year after the fire and two years after the fire including post-fire practices (salvage logging, skid trails and check dams)) and two computation targets (streams: hillslope-channel SC; and check-dams: hillslope-outlet SC) were considered, obtaining 66 maps of SC at fine spatial resolution (2 m of cell size). Burn severity classes were estimated using Landsat-7 imagery and the dNBR index. The indices' output analysis included geomorphic (landscape units), mathematic (significance, percentiles and frequency distribution), fire (burn severity classes and unburnt areas) and sedimentological (measured specific sediment yield - SSY) criteria. The IC-Borselli and AIC were the most responsive approaches to the effects of fire on SC at catchment scale, whereas the IC-Persichillo was the most sensitive index to the increasing burn severities. The overlay between the fire severities and the geomorphic features appeared as a key aspect to understand the hydrological response at both the stream-system and outlet targets. We found a good and positive agreement between the measured SSY in the three check-dams and the changes in the estimated SC_OUTLET due to the fire, especially with the IC-Borselli and AIC. For a better implementation of post-fire programs, we recommend SC_OUTLET maps -from AIC- to assess sediment transport in streams, which is dominated by the deposition process, and SC_STREAM maps -from IC-Borselli and AIC- to place sediment control measures at hillslopes for intense rainfall events when effective sediment transport happens.