4.1 Ephemeral gully morphological parameters
This study combined remote sensing imagery and the SegNet model to analyze temporal and spatial changes of ephemeral gully morphological features at a watershed scale. In 1999, Jiang et al. (1999) analyzed the distribution characteristics of ephemeral gullies using W-B (White-Black) aerial photographs, identifying 4,495 ephemeral gullies in the Zhoutungou watershed. We found that the ephemeral gully numbers in the watershed decreased from 4,495 in 1999 to 1,153 in 2009 due to the ‘Grain for Green Project’ significantly improving vegetation coverage on the Loess Plateau, reducing slope soil erosion and inhibiting the formation and development of slope ephemeral gullies (Chen et al. , 2019; Dou et al. , 2020; Liang et al. , 2019). However, ephemeral gully numbers changed after 2009 due to (1) the frequent occurrence of extreme rainfall on the Loess Plateau increasing soil erosion (Hu et al. , 2019; Li et al. , 2022; Wanget al. , 2020b; Zhao et al. , 2021); (2) the gully land consolidation project (implemented in 2011 to increase arable land by excavating soil from the slopes on both sides of gullies, combined with simultaneous comprehensive gully prevention and control measures); despite reducing watershed erosion to a certain extent (Chen et al. , 2020; Han et al. , 2018; Jin, 2014; Jin et al. , 2019; Kang et al. , 2021), its associated engineering involving topsoil stripping and ridge construction damaged the slope vegetation (increased slope) and soil environment (loosened soil structure), forming new ephemeral gullies on the slope.
This study found that ephemeral gully lengths ranged from 5 to 230 m (average 48.39 m)—more than 82.35% ranged from 15 to 70 m—with an average growth rate of 1.66 m y–1 from 2009 to 2021. However, other studies reported ephemeral gully lengths ranging from 7.71 to 237 m (average 56.50 m), with 60% from 30 to 60 m (Cheng et al., 2006; Cheng et al., 2007; Jiang et al., 1999). The hilly and gully region of the Loess Plateau has a unique and large undulating terrain, prone to developing long ephemeral gullies. Compared with previous studies, our study revealed shorter ephemeral gully lengths, which may be due to ephemeral gullies being: (1) buried or cut off by road construction and the gully land consolidation project, or (2) greatly eroded by headward gully erosion or vertical wall collapse under the influence of extreme rainfall.