Preliminary assessment and comparison of sediment connectivity indexes in a small catchment area: A case study in Tuscany

The problem of soil erosion is a current issue, especially in hilly and mountainous areas where the driving force is surface runoff, able to mobilize large amounts of sediment that may be delivered to rivers. This process must be considered in a context of climate change, where the number of extreme rainfall events is observed to increase, and their temporal distribution is bound to concentrate in very short periods between long dry intervals of time. The potential soil erosion and degradation can be assessed through the study of the linkages between sources and sinks in a watershed, typically called sediment connectivity assessment. The evaluation of sediment connectivity allows to identify areas prone to the delivery of sediments through the catchment and local depressions that may disconnect land units. The quantitative assessment of sediment connectivity can be evaluated by the using of indexes (e.g. Index of connectivity – IC) with a pixel by pixel procedure in a GIS environment. It is also possible to differentiate structural connectivity, which is evaluated with a geomorphological view, and functional connectivity, which consider the forcing processes and antecedent conditions (e.g. soil moisture). In this study we applied two well established approaches for calculating ICs that use geomorphological and driving force approaches in a small catchment area located in Tuscany. The area was mapped using LiDAR technology to have a high resolution digital terrain model and was affected by recent extreme rainfall events. The application of these two approaches to a small catchment area permitted to concentrate in a nearly homogenous site in terms of land use and focus on geomorphological features that contribute to a connection or a disconnection of catchment’s land units with high spatial resolution. Furthermore, the use of two different indexes allows to study the topic of sediment connectivity and associated soil erosion potential from two points of view with, considering both land management and future warning systems development. Further activities are focusing on the validation of the connectivity indexes, at the rainfall event scale, by monitoring the river suspended sediment transport at the outlet of the watershed in order to relate GIS calculated indexes with the amount of sediments delivered in the riverbed during each rainfall event.