Sediment retention (SR) and sediment yield (SY) are regulatory ecosystem services that would change due to seasonal vegetation conditions in a region. This study aimed to investigate the SR and SY in the Tajan watershed in Mazandaran Province, northern Iran, and their relationships to seasonal vegetation changes during 2022. The sediment delivery ratio model was implemented to evaluate SR and SY for four seasons using the InVEST software. Their relationship with the vegetation indices was also evaluated using geographically weighted regression (GWR) in Arc GIS 10.7. The highest level of SR and SY was observed in the central and southern parts of the watershed, respectively. The total SR and SY were estimated at 229 Mt y-1 (471 t ha-1 y-1 on average) and 5.2 Mt y-1 (11 t ha-1 y-1 on average), respectively. The maximum and minimum of SR occurred in the spring and winter, respectively, whereas the opposite was true for SY. Regional forests retained the most sediment, whereas rangelands had the highest SY. The GWR results showed a significant and positive geographic correlation between SR and vegetation indices (0.76
