Response of ephemeral gully erosion to rainfall variability and its prediction model based on UAV survey in the black soil region

Ephemeral gullies are small channels formed by surface runoff. Although they can be temporarily eliminated by tillage, they tend to reappear in the same location. Ephemeral gully erosion is one of the main types of soil erosion in cultivated land across the black soil region of Northeast China and poses a serious threat to land productivity and crop quality. This study aims to investigate the interannual variation and spatial differences in ephemeral gully erosion and to develop a prediction model. In the black soil region of Northeast China, ephemeral gullies formed by rainfall runoff generally reach peak development by late September, at the end of the rainy season. However, the timing of autumn harvest varies across regions, making UAV-based measurements at a regional scale challenging during the autumn season. In contrast, early summer provides a more suitable survey window: by this time, annual rainfall has already accumulated enough to induce ephemeral gully development, while crop cover remains relatively low, allowing gullies to be clearly observed. Based on this rationale, UAV-based surveys were conducted over two consecutive years in the northern part of the typical black soil distribution zone, i.e., from June 28 to July 29 in 2022 and from June 8 to July 1 in 2023. High-resolution DSMs and imagery were acquired, with an accuracy of approximately 3.8 cm, to capture both the morphological characteristics of ephemeral gullies and their influencing factors. Rainfall factors were obtained from meteorological stations in each county of the study area, and the runoff volume flowing through each ephemeral gully was calculated using the curve number (CN) method. The results showed that:1) Ephemeral gully erosion intensity exhibited significant interannual variation influenced by rainfall differences. Among 66 survey units, the proportion of units with observed ephemeral gullies was 75.78% in 2022 and 74.24% in 2023. In 2023, the average liner density, areal density, and volumetric density of ephemeral gullies decreased by 31.67%, 36.72%, and 40.78%, respectively, mainly due to lower rainfall during the same period. 2) In 2022, 215 ephemeral gullies were identified in 50 survey units, while in 2023, 151 gullies were found in 49 units, totaling 366 gullies over the two years. The average gully length, area, and volume were 99.26 m, 232.92 m², and 35.76 m³, respectively. In 2023, the number of gullies was only 70.23% of that in 2022, and the average length, area, and volume were 83.94%, 76.45%, and 71.94% of those in 2022, respectively. 3) In 2022, 46 ephemeral gullies with complete catchment areas were identified, and 27 reappeared in the same locations in 2023. Whether a gully reformed was significantly influenced by differences in catchment area and runoff volume (P<0.05). Based on these 73 gullies, ephemeral gully development was found to be significantly affected by catchment area and runoff volume. The length, area, and volume of gullies exhibited significant power-law relationships with average catchment slope (S) and runoff (Q), leading to the development of prediction models (n= 73, P<0.05). These findings provide a valuable reference for the control of ephemeral gully erosion and the development of regional-scale gully prediction models.