Threshold of watershed partition in SWAT based on separating hillslope and channel sediment simulations

•Determining the partition threshold based on separating hillslope and channel phases.•Two newly indicators were proposed for determining the threshold in channel phase.•Threshold in channel phase for the mean sub-watershed area was 2.4% of watershed area.•The changes in channel sediment were mainly due to the changes in channel dimensions.•The thresholds for sediment in hillslope and channel phases were different. Exploring how watershed partition influences hillslope and channel sediment in the Soil & Water Assessment Tool is important to identify the partition threshold and improve the model application in soil erosion evaluation. However, the influences on the channel phase, and the partition threshold for the entire watershed sediment process have not yet been well addressed. The Xixi basin located in southeast China was taken as an example with thirteen partition levels. Based on the idea of separating the simulations of the hillslope and channel phases, the partition threshold for channel sediment routing was identified according to changes in the parameters of channel sediment routing and the spatial distributions of channel deposition under different partition levels with two newly proposed indicators. This allowed the partition threshold of the entire watershed sediment process to be determined by comparing the thresholds for channel and hillslope sediment. The results indicate that (1) watershed partition has a substantial influence on the spatial distributions of channel deposition, and increasing sub-watersheds decreases the spatial variations of channel deposition. However, the influences can be obscured at the watershed outlet by changing channel routing parameters. (2) The changes in channel sediment routing are primarily due to the sensitivity of the channel dimensions in the model to watershed partition, which alter the hydraulic radius until a partition threshold, at which point the mean sub-watershed size is 2.4% of the total watershed area. (3) The thresholds for hillslope sediment generation and channel sediment routing are different, and the threshold for effectively simulating the entire watershed sediment process mainly depend on the hillslope sediment phase with a mean sub-watershed size of approximately 1.6% of the total watershed area.