Influence of ephemeral gullies on the occurrence and development of permanent gullies in the Loess Hilly Gully Region of China

Ephemeral and permanent gullies are vital components of erosion gully system, and the existence of a significant correlation between them has become one of the critical scientific questions in the control of gully erosion on the Loess Plateau in the new era. This study aims to explore the effect of ephemeral gullies above the head of permanent gullies on the occurrence and development of the connected permanent gullies in the Loess Plateau. Three typical small watersheds were selected as the research area located in the upper, middle, and lower reaches of the Chabagou watershed in Zizhou, Shaanxi Province. This study systematically analyzes the current connectivity between ephemeral and permanent gullies in the hilly-gully region of Loess Plateau and the impact of this connectivity on the formation under extreme rainstorm and their further development were explored at a three-year time scale based on unmanned aerial vehicle（UAV） aerial photography and high-precision modern measurement technology. The results showed that: 1）The hillslopes with ephemeral gullies were more conducive to newly formed permanent gullies under the '7.26' extreme rainstorm in northern Shaanxi in 2017. A total of 43 newly formed permanent gullies occurred in the three small watersheds during the rainstorm, of which 67.44 % were connected with ephemeral gullies. 2）The existence of ephemeral gully in the upstream of permanent gully head had an important influence on permanent gullies morphological characteristics. The permanent gullies connected with the ephemeral gullies were significantly longer than that disconnected with the ephemeral gullies, and the width of the permanent gullies was not significantly influenced. 3）Upslope ephemeral gullies also significantly affected the development of permanent gullies connected downslope. On the 3 a time scale after the ' 7.26 ' rainstorm in 2017, the permanent gullies connected with ephemeral gullies were more active in the subsequent development process than the ones not connected with ephemeral gullies. The proportion of gully head development in permanent gullies for newly formed permanent gully heads connected with ephemeral gullies was 1.67 times that of non-ephemeral gully-connected permanent gullies. and the gully headcut retreat rate was 1.80 times. For the existing permanent gullies, the above rates were 3.11 times and 2.17 times, respectively. This study can provide a theoretical basis for more effective management of gully erosion in the hilly-gully region of Loess Plateau.