Investigating sediment connectivity of a small catchment on the Loess Plateau using an appropriate index at an optimal spatial resolution

Abstract

Sediment connectivity provides an effective geomorphological and hydrologic tool in understanding the impact of morphological complexity and anthropogenic modification on the spatial transfer of sediment fluxes within a geomorphic system. However, sediment connectivity studies presented numerous challenges in complicated topographic area, such as the hilly-gully loess region of Chinese Loess Plateau, primarily due to the limited knowledge on the appropriate resolution and weighting factor used for deriving connectivity. In this study, the sediment connectivity was evaluated at six airborne LiDAR-derived DEMs (between 0.25 and 5 m pixel sizes) to identify the optimum resolution. Then the performance of Index of Connectivity (IC) with different weighting factors was evaluated based on field connectivity index (FIC) and the response to the anthropogenic activities. Results showed that 1) the linear features and details of pathways and terrace could not be distinguished clearly with the pixel size above 0.75 m. The flow paths were misaligned in locations with rapidly changing topography when the pixel sizes larger than 0.75 m while these paths were extremely complex and inconsistent with field investigation at a resolution of 0.25 m. The 0.5 m pixel size was demonstrated as the proper spatial resolution for the IC calculation in the study catchment. 2) The relationships between FIC and five IC versions at 0.5 m resolution presented significant correlations (p < 0.01), with the revised index of sediment connectivity (RIC) presenting the strongest correlation (R² = 0.51, p < 0.01). All the IC versions could reflect the effect of large-scale engineering disturbance (e.g., terrace, road) on sediment connectivity, whereas the connectivity index based on relative smoothness (IC_RS) and RIC better characterize IC variations under small scale disturbance in complex terrain. 3) The sediment connectivity showed spatial heterogeneity in different times, with the high IC values mainly concentrated in the gully channels with runoff pathway and the gully slope with sparse vegetation and steep slope. This study provided scientific reference for sediment connectivity evaluation in topographically complex area.