Impedance influence on the index of sediment connectivity in a forested mountainous catchment Franciele Zanandrea ⁎, Gean Paulo Michel, Masato Kobiyama Instituto de Pesquisas Hidráulicas (IPH), Universidade Federal do Rio Grande do Sul, 91509-900 Porto Alegre, RS, Brazil abstract article info Article history: Received 19 August 2019 Received in revised form 14 November 2019 Accepted 14 November 2019 Available online 19 November 2019 Keywords: Sediment connectivity Relative Smoothness Sediment and water impedance Forest 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 pro- mote (upslope area and slope) or reduce connectivity (downslope distance and impedance). Among these fac- tors, 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 km 2 ) 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 (W RS ), the USLE/RUSLE C-factor (W C ), 1-n (W n ), and the stan- dardized roughness index (W RI ). The higher values of IC with W C were observed in the high plain areas that are physically less connected to the catchment outlet. The use of W RS 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 W RI and W n as the impedance factor could not represent the significant role of forests in the sediment retention. Thus, the results showed that the W RS can represent vegetated regions, especially dense forests. © 2019 Elsevier B.V. All rights reserved. 1. Introduction Connectivity is a key issue for understanding sediment dynamics in the catchment and it is treated as an emergent property of the system, which represents landscape continuity over time (Heckmann et al., 2018; Wohl et al., 2019). According to these authors, connectivity de- scribes the degree to which a system facilitates the transfer of material (i.e., sediment) through itself and along with the landscape components such as hillslopes and river networks. Connectivity has been evaluated through its components, which are classified as structural and/or functional. Structural connectivity represents the spatial configuration related to pathways (continuity), length and flow resistance (Wainwright et al., 2011; Bracken et al., 2013). Such configurations result from roughness and terrain morphology. Functional connectivity is implicit to the real transfer of water and sediment and is related to surface runoff (Wainwright et al., 2011). Structural connectivity influences the thresholds of functional connectivity, which, in turn, modifies the geomorphic characteristics used for structural connectivity evaluation. For example, the larger the water depth generated by runoff, the smaller the hydraulic roughness effect, and consequently the smaller the impedance related to structural connectivity (Gumiere et al., 2011). Among the several methods to evaluate sediment connectivity, the use of geomorphometric indexes, which make it possible to detect areas more prone to sediment transfer, has been increasing (Borselli et al., 2008; Cavalli et al., 2013; Persichillo et al., 2018; Llena et al., 2019; Zanandrea et al., 2019). These indexes allow the evaluation of the degree of the linkage of sediment through landscape units (Cavalli et al., 2013). Flow resistance, also called impedance, is important for assessing con- nectivity, and it is evaluated in different ways using information on land use and topography. Borselli et al. (2008) proposed the connectivity index (IC) to estimate sediment connectivity at catchment scale. This index is calculated by using only physical properties of the catchment, disregarding the runoff characteristics (Persichillo et al., 2018), therefore enabling the determination of structural connectivity. The original IC uses the USLE/RUSLE C-factor proposed by Wischmeier and Smith Geomorphology 351 (2020) 106962 ⁎ Corresponding author. E-mail address: franciele.zanan@gmail.com (F. Zanandrea). https://doi.org/10.1016/j.geomorph.2019.106962 0169-555X/© 2019 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph