Flow Connectivity in Active Volcanic Areas: Use of Index of Connectivity in the Assessment of Lateral Flow Contribution to Main Streams

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Connectivity analysis is an important geomorphological and hydrologic tool that can be used to identify spaces that are prone to removal of primary sediments which are eventually assimilated into granular flows and related epiclastic processes. Studies of connectivity have been made in various areas, but to date, not in active volcanic areas, where such studies can be very useful due to the constant presence of loose volcanic material easily removed by epiclastic processes. Mobilization of loose pyroclastic sediments can trigger phenomena such as lahars, which are among the most dangerous in nature.

In this study, the index of connectivity (IC) (Borselli et al. 2008) was calculated by obtaining a weight coefficient (W) combining two elements: the coefficient C related to the universal soil loss equation and the revised universal soil loss equation (USLE-RUSLE) proposed by Borselli et al. (2008) for areas with vegetation cover or crops, and the roughness index (RI) developed by Cavalli et al. (2013), which characterizes bare soil areas. Combining both methods, we propose a new joint index of connectivity (ICJ) that does not overestimate the degree of connectivity in bare areas, while areas with vegetation cover are characterized based on their wellrecognized hydrologic impedance properties. This methodology may enable better characterization in highly dynamic environments such as active volcanic areas.

We also propose a new lateral hydrological efficiency index (LHEI) that increases the ability to identify watersheds that supply major amounts of sediment to main streams in ravines.

The application of this methodology in the active volcanic area of Volcán de Colima, the most active volcano in Mexico, is of great importance, because of the constant supply of new pyroclastic material from the top of the edifice, the high dynamicity of geomorphological processes, and the widespread presence of bare soil areas consisting of loose materials easily sourced, or assimilated, into epiclastic processes.