Temporospatial variations of structural sediment connectivity patterns in Taham-Chi watershed in Zanjan province, Iran

Overuse of natural resources exacerbates soil loss and sediment yield and has caused exceeding allowable rates of soil erosion. Many on-and off-site problems occurred due to irregular human use of natural resources leading to increasing sediment loads in the watersheds. Intensification of sediment yield leads to many intra and extra problems in the watersheds. The awareness on important and susceptible areas of sediment transport is therefore an essential tool to design effective strategies in management of sediment yield and transport in watersheds. Sediment connectivity is one of the most important concepts in this context to investigate sediment transfer between different locations and potential transfer of sediment particle across (between hillslopes, between channel and hillslope, within channel) a watershed. This research was therefore planned to investigate spatial and temporal variations of structural sediment connectivity in 1990, 2001 and 2014 years in Taham-Chai Watershed in Zanjan province, Iran.
In this study; slope, flow direction and flow accumulation layers have been made using digital elevation model (DEM) in 1:25000 scale and 100 m2 cell sizes. The C factor layer has been made using Landsat images in 1990, 2001 and 2014 years based on the NDVI. All layers along with the main stream network layer of the watershed have been used under two scenarios viz. the potential connection of sediment between hillslopes and catchment outlet and sediment connection between hillslopes and main channels to estimate structural sediment connectivity.
The results showed that the structural sediment connectivity based on the first scenario varied from -8.24 to -8.06 and 1.02 and 1.18 for min and max values, respectively. The Structural sediment connectivity index has decreased from -5 to -7 during 1990 until 2014. Regarding structural sediment connectivity maps, connectivity is high in western, southwestern and northwestern sections than other parts of the study area. The net spatial difference of structural sediment coactivity index values has been changed in the range of 0.9 from 2001 until 2014 which it is decreasing in the most sections of the study area and led to a decrease in structural sediment connectivity index. The results of the second scenario showed that the north and southwestern sections of the watershed along with central sections have low structural connectivity because of far distance from the channel network and slight topography respectively which these results are also consistent with results of first scenario. In addition, south, eastern and northeastern sections of the watershed have formed integrated hillslopes which they are prone to transfer sediment to the channel network.
Overall, on the basis of the results there is a possibility to investigate structural sediment connectivity and its temporal and spatial variations in the watershed. It also helps distinguish homogeneous sections by similar potential in sediment transport. Accordingly, investigation of structural sediment connectivity causes making conservative measurements and purposeful managerial decisions about sediment yield and transfer with respect to priority of homogeneous sections and financial and equipment constraints especially in countries such as Iran.