Susceptibility Assessment of Debris Flow Hazards from the Perspective of Watershed Units Grounded in the Random Forest (RF) Model — A Case Study in the Upper Reaches of the Fujiang River

As a critical ecological barrier in the upper Yangtze River basin, the upper reaches of the Fujiang River face severe constraints on sustainable development as a consequence of frequent debris flow hazards. For this reason, this study utilized 685 watershed units as evaluation units in the region to accurately identify high-risk areas and core driving factors while establishing a scientific susceptibility assessment framework. Eleven hazard-inducing factors, including watershed area, average elevation, and watershed elevation difference, were integrated to construct a Random Forest (RF) model on the Python platform for debris flow susceptibility assessment and key factor diagnosis. We ultimately arrived at the following conclusions. (1) Watershed area and mean annual precipitation are the primary factors controlling debris flow development, with contribution rates of 0.109 each, followed by the Normalized Difference Vegetation Index (NDVI) and average elevation (both exceeding 0.104), while soil type has the lowest contribution rate (0.032); (2) The RF model demonstrates excellent evaluation accuracy, with an Area Under the Curve (AUC) value of 0.82 and an Accuracy (ACC) value of 0.82. The study area is classified into five susceptibility levels, namely, very low, low, moderate, high, and very high. Distributing in a zonal pattern along the slopes of the main stem of the Fujiang River and its tributaries, such as the Huoxi River and Baicao River, the very high susceptibility areas (579.04 km²) and high susceptibility areas (58.1 km²) are primarily concentrated in unstable valley regions in the northwestern part of Maoxian-Beichuan, the eastern part of Songpan County, and the southeastern part of Pingwu County; (4) Among the 144 debris flow hazard sites within the watersheds, 56.94% (82 sites) are located in very high susceptibility areas, and 27.78% (40 sites) are in high susceptibility areas, validating the reliability of the assessment results. This study innovatively employs watershed units as evaluation units, thereby overcoming the limitations of traditional grid and administrative units. It clarifies the spatial pattern and key driving mechanisms of debris flow susceptibility in the upper reaches of the Fujiang River, providing a scientific basis and technical support for precise disaster prevention and control, land-use planning, and ecological protection in mountainous regions of southwest China.