The effects of land use and topographic changes on sediment connectivity in mountain catchments

Understanding the evolution of sediment connectivity associated with different land use and topographic changes is a prerequisite for a better understanding of sediment budgets and sediment transport processes. We used the Index of Sediment Connectivity (IC) developed by Cavalli et al. (2013) based on the original approach by Borselli et al. (2008) to study the effects of decadal-scale land use and topographic changes on sediment connectivity in mountain catchments. The input variables of the IC (i.e. land cover and topography) were derived from historical aerial photos using Structure from Motion-Multi View Stereo algorithms (SfM-MVS). The method was applied in different sub-catchments of the Upper River Cinca Catchment (Central Pyrenees), representative of three scenarios: (a) Land cover changes; (b) Topographic changes in agricultural fields (terracing); and (c) Topographic changes associated with infrastructure (road construction). In terms of land cover changes, results show that although connectivity is increased in some areas due to the establishment of new field crops, for most of the study area connectivity decreased due to afforestation caused by rural abandonment. Topographic changes due to the establishment of agricultural terraces affected connectivity to a larger degree than land cover changes. Terracing generally reduced connectivity due to the formation of flat areas in step-slopes, but in certain points, an increase in connectivity caused by the topographic convergence produced by terraces was observed. Finally, topographic changes associated with road construction greatly modified surface flow directions and the drainage network, resulting in changes in connectivity that may affect erosional processes nearby. The methodology used in this paper allows to study the effects of real decadal-scale land use and topographic changes on sediment connectivity and also evaluating and disentangling those changes. Furthermore, this approach can be a useful tool to identify potential risks associated with morphological and land use changes, involving road infrastructures.

This research was carried out within the framework of two research projects funded by the Spanish Ministry of Economy and Competitiveness and the European FEDER funds: MORPHSED (CGL2012-36394) and MORPHPEAK (CGL2016-78874-R). The first author has a grant funded by the Ministry of Education, Culture and Sports, Spain (FPU016/01687). The second author is a Serra Húnter Fellow at the University of Lleida. This manuscript has benefited from the work carried out during a Short Term Scientific Mission funded by the EU Cost Action Connecteur (ES1306: Connecting European Connectivity Research), and all discussions during the different meetings the Working Group 2 and 4 of this Action organised. The first and second authors are part of the Fluvial Dynamics Research Group-RIUS, which is a Consolidated Group recognized by the Generalitat de Catalunya (2017 SGR 459645). We also acknowledge the support of the CERCA Program of the Generalitat de Catalunya.