A graph theory tool for assessing structural sediment connectivity: Development and application in the Mercurey vineyards (France)

In agricultural lands, assessing how the spatial patterns of landscapes affect sediment connectivity is a key factor in understanding the nonlinear behavior of sediment transfer to the outlet. Important questions remain regarding this relationship. For example, to what extent is sediment travel disconnected from the flows that transport sediments? What connections exist between the components of the sediment cascade? The answers to this methodological problem are particularly important to land managers and farmers. In this paper, we further explore graph theory as a tool for assessing structural sediment connectivity at the catchment scale. Our approach combines two graph theory indices, the accessibility index and the flow index, to create the residual flow (RF) index, which is used to assess structural sediment connectivity. We apply the method to the case study of Mercurey (Burgundy, France), where agricultural activities (i.e., wine growing) have resulted in environmental issues, such as soil loss and flooding. In this area, vine-growers have developed specific structures (such as concrete pits, drainage ditches or curved paths) to collect and trap sediments that have been eroded from vine plots, which can then be used to backfill parcels upslope. These man-made structures modify the spatial pattern of the sediment cascade. The indices mentioned above can be used to highlight and quantify these changes. In particular, the influence of linear features (such as roads and hedges) and artificial sinks can be determined. Additionally, we find that unexpected behaviors of the sediment cascade system can result from dysfunctions in the management structures. The RF index shows promising results as a tool for rapidly assessing structural connectivity at the catchment scale, although further methodological advances will help ensure reproducibility.