Ephemeral gully erosion is a primary mode of soil erosion that is highly visible, affecting soil productivity and restricting land use. Watershed is the basic unit of soil erosion control; existing research has focused on several typical ephemeral gullies or slopes, which do not fully display changes in ephemeral gullies at a watershed scale. This study analyzed the spatial-temporal evolution and development rate of ephemeral gully erosion at the watershed scale on the Loess Plateau from 2009 to 2021 using remote sensing images (0.5 m resolution), unmanned aerial vehicles (UAV), and field investigations. The results revealed that: (1) most ephemeral gullies occurred in southwestern parts of the watershed, with many hills and large slope gradients; (2) average growth rates of each ephemeral gully frequency, length, density, dissection degree, and width were 2.87 km 2 y –1, 1.66 m y –1, 0.12 km km –2 y –1, 0.0125% y –1, and 0.04 m y –1 , respectively; (3) ephemeral gully erosion volume ( V) and length ( L) had a good power function relationship: V = 0 . 0842 L 1 . 1932   ( R 2 = 0 . 80 ) . The root mean square error (RMSE) and coefficient of determination (R 2) between the measured and predicted ephemeral gully volumes suggest that the V–L relationship has a good predictive ability for ephemeral gully volume. Thus, the V–L model was used to evaluate the development rate of ephemeral gully erosion volume in small watersheds from 2009 to 2021, revealing an average value of 743.20 m 3 y –1. This study proposed a feasible model for assessing ephemeral gully volume and volume changes at a watershed scale using high-resolution remote sensing images, providing a reference for understanding the development of ephemeral gully erosion in small watersheds over time.

Gully erosion is one of the main modes of slope erosion on the Loess Plateau, which plays a connecting role in the slope gully erosion system. The Loess Plateau has wide and densely distributed gullies. The study selected a typical small watershed in the hilly and gully region of the Loess Plateau to measure the morphological characteristics and spatial-temporal distribution of gullies. A deep learning image semantic segmentation model was used to identify and extract the morphological features of gullies at the watershed scale from 2009 to 2021 based on remote sensing images (0.5 m resolution) and then analyze their temporal and spatial distribution characteristics. The results revealed that: (1) most gullies occurred in the hilly southern parts of the watershed, which has complex landforms and large slope gradients; (2) gully number increased from 1,159 in 2009 to 2,312 in 2021 (average 97 per year), with a frequency development rate of 2.87 km –2 y –1; (3) gully length generally ranged from 25–40 m, with an average growth rate is 1.66 m y –1 and density development rate of 0.12 km km –2 y –1; (4) gully width ranged from 0.5–1.5 m, with an average growth rate of 0.04 m y –1. (5) the total gully area increased from 0.0566 km² in 2009 to 0.1072 km² in 2021, with a development rate of 4,213.39 m² y –1 and dissection degree development rate of 0.0125% y –1. This study provides a theoretical and scientific basis for gully erosion control and eco-environmental protection at the watershed scale on the Loess Plateau.