Rainfall-Pattern-Dependent Regulation of Hillslope Erosion by Vegetation Conservation Measures in Subtropical Hilly Farmland: A Multi-Method Analysis

The Southern Jiangsu hills region is located within the Yangtze River Delta Ecological Barrier. Soil erosion poses a threat to the development of commercial forests and the water quality of Lake Taihu, making it urgent to quantify the mechanisms by which vegetation and rainfall regulate slope erosion. This study established five standard runoff plots in Zhangzhu, Yixing, and continuously monitored runoff and soil loss from 2022 to 2023. By combining Random Forest modeling, partial least squares structural equation modeling (PLS-SEM), and moderation effect analysis, the study assessed the contributions of various driving factors under different rainfall types. The results indicate that the peach orchard (PEA + GRA) is an optimal ecological model, achieving runoff and sediment reduction rates of over 54.09% and 70.19%, respectively. Rainfall is the dominant factor driving runoff (r = 0.75), while the maximum 30 min rainfall intensity (I30) is the dominant factor driving soil loss (r = 0.82). Furthermore, during low- to moderate-intensity rainfall events, vegetation attributes primarily govern hydrological responses; however, during extreme Type III rainstorms (24 h rainfall exceeding 50 mm), rainfall volume becomes the decisive factor. Moderation analysis further reveals that vegetation height and cover exert significant moderating effects on the initial transition phase from rainfall to runoff. These findings provide evidence-based guidance for optimal soil and water conservation strategy selection in subtropical hilly landscapes.