Estimating functional connectivity from DEMs of difference to evaluate a connectivity index

Sediment connectivity is an important system property governing sediment delivery and sensitivity to changes with respect to sediment transfer. While structural connectivity is related to the composition and configuration of a geomorphic system, functional connectivity is effected by the activity of geomorphic processes that erode, transport and deposit sediments within and out of the area of interest. A range of connectivity indices have been proposed for the assessment of structural connectivity; in many applications, however, we are interested in functional connectivity. Hence, the predictive or explanatory capacity of connectivity indices with respect to functional connectivity is an important research need that has been addressed in few studies only. In this study, we evaluate the predictive or explanatory capacity of the most frequently used index of sediment connectivity by correlation with the sediment delivery ratio (SDR) on lateral moraine sections in multiple Alpine study areas. The SDR describes the proportion of gross erosion that has been exported from an area of interest (=sediment yield); notwithstanding conceptual issues, it can be regarded as a quantitative indicator of functional sediment connectivity. We derive (net) erosion, sediment yield and SDR using digital elevation models of difference computed from airborne LiDAR surveys or photogrammetric analysis of aerial photos. Besides, we explore the effect of assuming different bulk densities for freshly eroded vs. deposited sediments on the computations. Results suggest a positive correlation of intermediate strength between the connectivity index and functional connectivity as measured by SDR, corroborating the value of index-based connectivity assessments in studying sediment cascades, delivery and budgets.