Impedance influence on the index of sediment connectivity in a forested mountainous catchment

Because connectivity is a key issue for understanding sediment dynamics in catchments, the index of sediment connectivity (IC) can be an important tool for hydrosedimentological studies. The IC depends on factors that promote (upslope area and slope) or reduce connectivity (downslope distance and impedance). Among these factors, the estimation of impedance is the one that offers more obstacles, mainly related to its dependence on characteristics difficult to assess (i.e., the effect of vegetation). The present study aims to propose a new approach to calculate the weighting factor of impedance (W), used in the IC calculation, and to compare its performance with three other approaches by constructing IC maps for a forested catchment. The new approach uses an index called Relative Smoothness (RS), based on the Manning coefficient (n) that preserves the dimensionlessness of the Index. The study area is the Mascarada River catchment (320 km²) in Brazil. About 70% of the total catchment area is covered by forests. Impedance maps with four different procedures and their respective IC maps were generated and compared for the study catchment. The four different procedures for impedance estimation included the use of RS (WRS), the USLE/RUSLE C-factor (WC), 1-n (Wₙ), and the standardized roughness index (WRI). The higher values of IC with WC were observed in the high plain areas that are physically less connected to the catchment outlet. The use of WRS in the IC demonstrates lower IC values in the high plain areas of the upstream catchment and higher IC values in landslide scars. Furthermore, the forested areas presented lower IC values when compared to other methodologies. The use of WRI and Wₙ as the impedance factor could not represent the significant role of forests in the sediment retention. Thus, the results showed that the WRS can represent vegetated regions, especially dense forests.