Background Soil erosion is one of the serious phenomenon during the soil degradation process, and it directly affects the ecological environment and local economic development. The quantitative characteristics of the gully, which is a principal component factor for assessing the soil erosion intensity, plays an important role in saving water and soil conservation study.

Methods Based on the DEM data at 30 m resolution, this paper used the Universal Soil Loss Equation (USLE) and the ArcGIS Hydrological Analysis Module to analyze the gully quantitative characteristics and soil erosion status of the typical secondary tributary watershed in the Yellow River Basin. According to the yearly soil erosion modulus, four grades were divided by using the occurred frequency of the annual modulus. Then, typical years were selected by using closest to the average soil erosion modulus on each grade. Finally, the relationship between soil erosion amount and gully quantity characteristics (gully length, basin area, and gully density) at the different grades was analyzed.

Results The gully length was distributed in 0.26-154.41 km with an average of 9.41 km while the basin area was distributed in 0.73-177.03 km² with an average of 10.57 km². Gully density was distributed in 0.36-2.34 km/km² with an average of 0.94 km/km². The variation of soil erosion modulus during the study period (2000-2017) was from 11.84 to 72.83 t/(hm²·a), and showed a fluctuation increasing trend. Frequency analysis indicated there were four grades as 0-20, 20-30, 30-50 and 50-80 t/(hm²·a), and best match years were 2015, 2006, 2004 and 2016, respectively. In these typical years, soil erosion amount increased significantly with the increase of gully length and basin area, and the gully length of any typical year has a faster regression response rate than the basin area. Soil erosion amount changed irregularly with the increase of gully density.

Conclusions The gully length and basin area were mainly concentrated in < 10 km and < 10 km², which accounted for 80.11% and 78.45% of the total, respectively. The density of gully was mainly concentrated between 0.5-1.5 km/km², which accounted for 92.82% of the total. There was a significant linear regression relationship between the soil erosion amount and the gully length and basin area (Sig. < 0.001). The erosion modulus of different grades had the same pattern. Therefore, the soil erosion amount of the corresponding watershed can be obtained more simply and directly by gully length or basin area. On the whole, there was no significant linear regression relationship between gully density and soil erosion amount, but there was a higher amount of soil erosion than the overall level when the gully density between 0.74-0.98 km/km², and its mean value was 6 times as much as the others.