Analysis of sediment accessibility and availability concepts based on sediment connectivity throughout a watershed

The identification of the degree of linkage between sediment source and sink areas and also the channel network allows for prioritizing areas for sediment control measures at the watershed scale. The present study was therefore planned to investigate sediment connectivity based on structural and functional aspects. To that end, the index of connectivity (IC) and sediment delivery ratio (SDR), respectively, were calculated based on the approaches of Borselli, Cassi, & Torri (2008) and a sediment contribution area (SCA) representing structural and functional sediment indices of connectivity. Results showed that high structural connectivity in an area does not necessarily result in high transportation of sediment, since structural connectivity only reflects the potential for sediment transfer between specific source and sink areas. We name this potential movement as sediment accessibility. Meanwhile, high functional sediment connectivity occurs only where soil erosion processes are actively eroding sediment, and consequently we name this condition as sediment availability. Therefore, four sediment connectivity states can occur: (a) sediment availability and sediment accessibility (SASA); (b) sediment availability and sediment inaccessibility (SASIA); (c) sediment unavailability and sediment accessibility (SUASA); and (d) sediment unavailability and sediment inaccessibility (SUASIA). From the application of this concept in the Taham representative watershed in Zanjan Province, Iran, there was a consistency between both structural and functional connectivity in seven subwatersheds within the study area, with one exception with higher sediment availability than sediment accessibility. Such information in combination with a sediment fingerprinting approach assists managers to plan appropriate approaches for achieving the desired sediment (dis)connectivity.