Topographic Modulation of Extreme Precipitation-Driven Rainfall Erosivity in the Hengduan Mountains

Extreme precipitation can disproportionately enhance rainfall erosivity in complex mountainous terrain, yet its spatial amplification and topographic differentiation remain poorly understood. Focusing on the Hengduan Mountains, this study evaluated three precipitation products (ChinaMet, CHM_PRE, and IMERG) against station observations and assessed their ability to capture precipitation extremes. Using the best-performing product, rainfall erosivity associated with total (PRCPTOT), heavy (R95p), and extreme (R99p) precipitation was estimated for 2005–2024, and its spatial patterns, amplification effects, topographic differentiation, and hotspots were analyzed. CHM_PRE showed the best overall performance, with a correlation coefficient (CC) of 0.83 and a Kling–Gupta efficiency (KGE) of 0.74, together with the highest probability of detection (POD = 0.95), accuracy (ACC = 0.83), and critical success index (CSI = 0.78) for extreme precipitation. Precipitation and the corresponding rainfall erosivity exhibited a pronounced southeast-to-northwest decreasing gradient. Although R95p and R99p accounted for only 9.61% and 2.43% of total precipitation, they contributed 14.84% and 4.33% of total rainfall erosivity, yielding erosivity amplification factors (AFs) of 1.52 and 1.73, respectively. This indicates a disproportionate contribution of precipitation extremes to rainfall erosivity, with stronger amplification under R99p. Rainfall erosivity also exhibited pronounced topographic differentiation, and high-level hotspots were consistently concentrated along the southeastern and southern margins. Extreme hotspots under PRCPTOT and R95p occurred at mean elevations of 2735.19–2791.92 m and mean slopes of 14.79–15.09°, whereas R99p intense hotspots occurred at a mean elevation of 2374.15 m and a mean slope of 12.28°. Strongly undulating mid-high mountains were the dominant geomorphic units within PRCPTOT and R95p extreme hotspots, while moderately and strongly undulating mid-high mountains dominated R99p intense hotspots. Moreover, hotspots became increasingly localized as precipitation extremity increased. These findings highlight the disproportionate erosive significance and spatial selectivity of precipitation extremes and provide a basis for identifying priority areas for soil and water conservation and rainfall-related hazard management in the Hengduan Mountains under climate change.