Division, Naming, and Spatial Distribution Characteristics of Small Watersheds in Mixed Terrains:A Case Study of Yingwen River Basin

[Objective] Focusing on the Yingwen River Basin in Shandong Province, this study aims to explore the extraction, division, naming methods for small watersheds in mixed terrains, and to analyze spatial distribution characteristics of small watershed areas and river network densities. [Methods] This study utilized high-precision SRTM-DEM data, combined with hydrological analysis modules, to extract micro-watersheds, river networks, and watershed boundaries. The optimal catchment threshold was determined using the river network density method. Micro-watersheds were merged into small watershed units based on natural catchment relationships, and named according to Specification SL 653—2013. In complex terrain areas, local corrections were made by referencing high-resolution remote sensing images and field survey results to ensure the accuracy of the division. Meanwhile, the main stream, first-order tributaries, and second-order tributaries of the Yingwen River Basin were extracted, and the characteristics such as the area of small watersheds and the density of river networks were statistically analyzed. [Results](1) This study successfully extracted 1 main stream (the main stream of Yingwen River), 22 first-order tributaries, 48 second-order tributaries, and 1 large reservoir, establishing a complete water system network. (2) A total of 28 small watersheds were merged. Small watershed areas in upstream mountains were generally smaller, while those in downstream hilly plains were relatively larger. This was mainly caused by terrain limitations combined with human activities. (3) All 28 small watersheds were named based on the proposed naming process, providing strong support for watershed management and water resource protection. (4) The river network density in Yingwen River Basin exceeded 0.2 km/km2, indicating abundant water system resources. Areas with river network densities of 0.4-0.6 km/km2 accounted for 60.71% of the total area, mainly concentrated in the middle and lower reaches. Areas with densities of 0.6-0.8 km/km2 accounted for 28.57%, distributed in the upper, middle, and lower reaches. [Conclusion] (1) The analytical framework for division, naming, and spatial distribution characteristics of small watersheds in mixed terrains proposed in this paper provides scientific methods and practical cases for similar studies. (2) Complete-type small watersheds are dominant, indicating relatively intact water systems with limited human disturbance. The spatial distribution characteristics of small watershed areas and river network densities reveal the combined influence of terrains and human activities on water system development. (3) The number of first-order tributaries in the Yingwen River Basin exhibits a significant increasing trend from upstream to downstream, reflecting the complexity of river network systems in middle and lower reaches and the promoting effect of human activities on water system development. The distribution characteristics of the length of tributaries per unit area and the density of river networks further reveal the abundance and spatial differences of water system resources within the basin.