Objectives Gully erosion has become a critical issue in the mollisol region of Northeast China, severely degrading black soil resources and threatening national food security. Ephemeral and permanent gullies are the main erosion gullies in this region. Current gully control strategies mainly focus on treating permanent gullies, while insufficient attention has been paid to preventing the transformation of ephemeral gullies into permanent gullies, largely due to a limited quantitative understanding of this process. This research aims to offer a quantitative analysis of the development of ephemeral gullies into permanent gullies in recent years within a typical watershed in the mollisol region of Northeast China.

Methods The Hebei watershed in Nenjiang County, Heilongjiang Province, with an area of 28.02 km², was selected as the study area. Morphological parameters of ephemeral and permanent gullies from 2011 to 2022 were extracted through visual interpretation of high resolution satellite images and unmanned aerial vehicle (UAV) imagery. The interpretation accuracy was validated using field measurement data obtained by global navigation satellite system real-time kinematic surveys. Slope and drainage area were derived from a 1 m resolution digital elevation model generated from UAV imagery. Slope and drainage area data were plotted on a double logarithmic coordinate system to construct a slope-area critical terrain threshold model for gully transformation.

Results From the analysis of the phenomenon of ephemeral gullies developing into permanent gullies, the development rate, distribution of vulnerable areas, and critical terrain parameters were acquired. The results showed that: (1) From 2011 to 2022, a large number of ephemeral gullies in the study area developed into permanent gullies, accounting for 23.7% of the total ephemeral gully length in 2011. By 2022, the number and total length of permanent gullies increased by factors of 5.5 and 5.0, respectively, compared with those in 2011. Approximately 50% of the permanent gullies originated from ephemeral gullies during the study period, with a length change rate of 16.6 m/(km²·a); (2) The development of ephemeral gullies into permanent gullies was mainly distributed intermittently in the lower and middle reaches of both banks and in downstream areas of gully heads within sub-watersheds, with a few occurring along both sides of the main channel; (3) A slope-area critical threshold model for gully transformation was established as S≥0.421A^-0.252, achieving an overall prediction accuracy of 72.7%.

Conclusions High resolution satellite imagery combined with high precision modern surveying and mapping techniques proved effective for identifying and quantifying the transformation of ephemeral gullies into permanent gullies. Ephemeral gullies were rapidly developing into permanent gullies in the Hebei watershed in the mollisol region of Northeast China. The proposed critical terrain threshold model can effectively predict areas vulnerable to gully transformation. Further research incorporating additional controlling factors and different erosion environments is needed to improve the accuracy and applicability of the model.