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Determining the soil quality of alluvial fans and its influencing factors is essential for understanding, utilizing, and protecting alluvial fans. In this study, a soil quality index (SQI) was constructed, and the soil of river terrace farmland was used as a control to evaluate the soil quality of alluvial fans in the Lhasa River Basin of the Qinghai-Tibet Plateau. Meanwhile, the influencing factors of soil quality of the alluvial fan were studied using correlation analysis. The results demonstrated that: 1) There is no significant difference in physicochemical indicators between the soil of the alluvial fan and river terrace farmland, namely the total nitrogen, total phosphorus, total potassium, available nitrogen, available phosphorus, available potassium, soil organic matter, pH, electrical conductivity, and soil erodibility; however, the coefficient of variation of each physicochemical indicator of the alluvial fan is greater than that of river terrace farmland. 2) The mean SQI value of alluvial fans and river terrace farmlands are 0.37 and 0.38, respectively. There is no significant difference between the values, demonstrating that the soil quality of alluvial fans is not lower than that of river terrace farmland. 3) The soil quality of alluvial fans has a highly significant positive correlation with vegetation coverage, land use, annual precipitation, altitude, and geomorphic divisions; a significant positive correlation with geomorphic location; and a highly significant negative correlation with annual temperature. Therefore, from the perspective of soil quality, the alluvial fan in the Lhasa River Basin can be used as a reserve arable land resource.

Characteristics of soil erosion under extreme rainfall conditions are indicators of the ecological restoration measures as well as the rationality of land use patterns in the region. To this end, 12 dam-controlled catchments were selected for investigation after an extreme rainstorm event in the northern Shaanxi Province on 25–26 July 2017 (known as the “7.26” rainstorm). Soil erosion intensities in the 12 catchments were obtained by excavations of sedimentation profiles and sedimentation area measurements. Digital orthophoto maps and digital terrain models by unmanned aerial vehicle were generated to characterise land use types and areas, slope gradients, and the distance from the land to the boundary of dam-land along the flow accumulation path. Stepwise regression analysis was used to determine the main factors affecting catchment erosion intensity. Results showed that the average sedimentation thickness and overall soil erosion intensity across the 12 catchments ranged from 0.16 to 1.67 m and 10295–49227 t km−2, respectively. Soil erosion caused by the extreme rainstorm was 10–50 times of the soil loss tolerance on the Loess Plateau (1000 t km−2 year−1). Stepwise regression analysis revealed that the more similar the shape of a catchment to a circle, the closer the distance between the slope-cropland and dam-land, the larger the erosion modulus within the catchment. Furthermore, the presence of a cement road along the valley shoulder greatly reduced the modulus of soil erosion. These findings indicate that the existing ecological conditions in the dam-controlled catchments were not able to effectively resist erosion under the extreme rainfall conditions of this event. Thus, optimising the distribution of land use types within catchments should be a focus for soil erosion control.