Terracing is an important measure to conserve water and soil on the Loess Plateau. Previous studies have showed that due to extreme rainstorms and a lack of maintenance, loess terraces have experienced degradation predominantly as collapses and sinkholes. Investigating the hydrological processes associated with terrace degradation is vital to understand terrace degradation mechanisms and maintain terraced landscape sustainability. Using a high-resolution digital elevation model (DEM) based on unmanned aerial vehicle (UAV)remote sensing, as well as SIMulated Water Erosion (SIMWE) model, we explored the runoff-sediment dynamics on the terraced slopes on the Loess Plateau under rainstorm conditions and the terrace degradation patterns. The results showed that the dominant longitudinal water flow between terrace steps and converging lateral water flow along terrace surfaces indicate terrace ridge collapses and terrace surface sinkholes, respectively. Damaged areas exhibit high sediment flux and erosion potential. The water dynamics and degradation pattern were influenced by the original slope topography and terrace morphology. Specifically, most damages are distributed on concave slopes with concentrated water flow. In narrow terraces, under scattered longitudinal water flow, the dominant damages are ordinary collapses and the overall terraced topography tends to degrade into natural slopes. In wide terraces, concentrated longitudinal and well-developed transverse water flows form a basic degradation pattern dominated by sinkhole-induced collapses. This study verified the feasibility of SIMWE-based hydrological simulation in assessing the degradation pattern of terraces on the Loess Plateau and demonstrated its potential for spatial scales and complex scenarios through compared with the soil moisture content (SMC)-based method. The study concluded that the dominant runoff paths under the constraints of slope microtopography control the terrace degradation patterns. Our findings can serve as a theoretical basis for predicting hydrological hazards on terraces on the Loess Plateau and conducting a scientific design of terraces. The dominant longitudinal waterflow between terrace steps and converging lateral waterflow along terrace surface control the degradation patterns in the abandoned terraces on the Loess Plateau, and the hydro-geomorphological response is constrained by original slope topography and terrace morphology. image

The black soil region of Northeast China is the largest commercial grain production base in China, accounting for about 25% of the total in China. In this region, the water erosion is prominent, which seriously threatens China's food security. It is of great significance to effectively identify the erosion-prone points for the prevention and control of soil erosion on the slope of the black soil region in Northeast China. This article takes the Tongshuang small watershed (Heilongjiang Province in China) as an example, which is dominated by hilly landforms with mainly black soil and terraces planted with corn and soybeans. Based on the 2.5 cm resolution Digital Elevation Model (DEM) reconstructed by unmanned aerial vehicles (UAVs), we explore the optimal resolution for hydrological simulation research on sloping farmland in the black soil region of Northeast China and explore the critical water depth at which erosion damage occurs in ridges on this basis. The results show that the following: (1) Compared with the 2 m resolution DEM, the interpretation accuracy of field roads, wasteland, damaged points, ridges and cultivated land at the 0.2 m resolution is increased by 4.55-27.94%, which is the best resolution in the study region. (2) When the water depth is between 0.335 and 0.359 m, there is a potential erosion risk of ridges. When the average water depth per unit length is between 0.0040 and 0.0045, the ridge is in the critical range for its breaking, and when the average water depth per unit length is less than the critical range, ridge erosion damage occurs. (3) When local erosion damage occurs, the connectivity will change abruptly, and the remarkable change in the index of connectivity (IC) can provide a reference for predicting erosion damage.