Ephemeral gully (EG) erosion is an important type of water erosion. Understanding the spatial distribution of EGs and other influencing factors at a regional scale is crucial for developing effective soil and water management strategies. Unfortunately, this area has not been sufficiently studied. The present study visually interpreted the EGs based on Google Earth images in 137 small watersheds uniformly distributed in the Loess Plateau, compared them with measured results, and analyzed the factors influencing EG formation and density using GeoDetector. The results showed that visually interpreting EGs from Google Earth images was suitable for EG regional studies. Out of the 137 small watersheds, 33.6% had EG occurrence with an average density of 3.41 km/km². Rainfall (R) and slope gradient (S) were the primary factors influencing the formation of EGs, while the area proportion of sloping farmland (APSF) and soil erodibility (K) were the main factors affecting EG density. The interaction of dual factors had a greater influence compared to single factors, with the interaction between S and Normalized Difference Vegetation Index (NDVI) having the greatest impact on EG formation and the interaction between K and NDVI on EG density. Although natural forces significantly influence whether EGs can form in a specific area, human activities greatly affect the density of the gullies that develop. This underscores the importance of proper land management in controlling gully erosion. These findings could provide theoretical support for EG prediction models and a scientific basis for soil and water loss control strategies at the regional scale.

Gully development is a significant geomorphological and environmental process that affects land degradation worldwide, with ephemeral gullies (EGs) and permanent gullies (PGs) being the two most common types. These two gully types are often spatially connected, and with such EG-PG connectivity can accelerate erosion. However, systematic research on this phenomenon remains limited, particularly at the regional scale. This study focuses on the spatial connectivity between EGs and PGs in the Songnen black soil region of northeast China. An unequal probability stratified sampling was used to establish 977 small watershed units, and a database of gullies and their connectivity was constructed based on sub-meter imagery. Among them, 55 representative units were randomly selected within geomorphic zones for field surveys and UAV validation to ensure data accuracy. Spatial patterns of gully connectivity were analyzed, and dominant controlling factors were identified using the Geodetector, which quantifies spatial stratified heterogeneity and evaluates the explanatory power of potential driving factors. The results are as follows: (1) Gully connectivity varies significantly across the region, with hotspot areas where more than 50% of permanent gullies are connected to ephemeral gullies, and cold spot clusters elsewhere. (2) Permanent gullies connected to ephemeral gullies differ significantly from unconnected ones in both length and width, with the former exhibiting a more elongated morphology. (3) Slope length and mean annual precipitation are the primary drivers of gully connectivity, both showing significant positive effects. Moreover, the interaction between mean annual precipitation and slope length shows the strongest explanatory power, indicating that precipitation, in combination with topographic features, plays a dominant role in shaping gully connectivity. By examining the spatial patterns of gully connectivity, this study contributes to a more refined understanding of gully morphological evolution and offers empirical insights for enhancing gully erosion models and optimizing regional soil and water conservation strategies.

Gully erosion represents one of the most severe forms of land degradation. In regional management decisions, gully density serves as a crucial metric. As an essential measure under China’s national strategy for protecting black soil, gully erosion control projects rely on accurate simulation of gully density at the regional scale to enable more efficient and precise management. Taking the northeast China’s Songnen typical black soil region as the study area, this research employed a stratified unequal probability systematic sampling method to select 977 small watershed sample units. Using sub-meter resolution Google Earth imagery, gully density on farmland was visually interpreted. To ensure the interpretation’s accuracy, 55 typical small watersheds were randomly selected as validation units for field investigations. On this basis, the RF algorithm was used with 13 selected factors to predict farmland gully length density. The results showed the following: 1) The Random Forest model demonstrated high accuracy and applicability, with an NSE exceeding 0.5. Residuals primarily centered around -0.1 km/km². 2) Slope was identified as the key influencing factor for farmland gully density, followed by multi-year average May precipitation, slope length, and multi-year average rainstorm volume. Threshold analysis revealed a significant increase in gully density when slope exceeded 1.21° and slope length surpassed 74.15 m, but a weakening effect was observed when the slope and slope length reached certain thresholds. 3) The prediction results indicated higher gully densities in low mountains and hilly areas. Regions with a density range of 0–0.05 km/km², followed by 0.05–0.25 km/km². As density increased, the proportion of area gradually declined, with areas >1 km/km² accounting for no more than 15% of the total. High-density regions were concentrated in low mountains and hilly areas, with average gully densities of 1.33 km/km² and 1.80 km/km², respectively, whereas low-density regions were concentrated in plains, with densities close to 0 km/km². This study provides theoretical and technical support for regional gully management decisions in the black soil region of Northeast China, contributing to the protection of black soil resources. Keywords: Permanent gully; Songnen typical black soil region; Random Forest; Regional scale; Gully length density