Occurrence and development of newly formed gullies under extreme rainstorm conditions using remote sensing images

Abstract: Soil erosion has posed some serious damage on the land resources in the world. The "Grain for Green" project in 1999 has been launched to restore the vegetation on the steep slope farmlands and control the soil and water loss for a better ecological environment of the Loess Plateau in China. But, the frequent extreme rainstorms have aggravated the occurrence and development of gully erosion under global warming in recent years. The development of the gullies after the extreme rainstorm still remains unclear so far. Fortunately, the Unmanned Aerial Vehicle (UAV) data can be collected fast, accurately, and efficiently after a rainstorm. The flexible acquisition time of UAVs can be expected to serve as an effective tool for the development of gully. Taking the Wangwugou Small Watershed of Chabagou Watershed on the Loess Plateau as the research area, this study aims to investigate the formation and development of gully under a rainstorm using UAV images. A case study selected the "7·26" extreme rainfall that occurred in Northern Shaanxi of China on July 26, 2017, with a daily rainfall of 206.6 mm. The patterns of newly formed gullies were captured under the climate. An equation with the topographic parameters was established for the occurrence and development of newly formed gullies in the following three years. The results showed that: 1) There were 45 newly formed gullies during the extreme rainstorm, which were about 101 gully/km2 in total. Four types were divided, including the hill slope, terraced field, unpaved roadway, and bottom gullies. There were more gullies found on the slopping hills. The bottom and terraced field gullies presented much wider and larger in the study area. The unpaved roads, check dam farmland, and sloping farmland were more prone to the gullies. 2) In the three years after the formation of the new gullies, the development of the new gully heads was faster than that of the original, 34.48% of which were further advanced, indicating 1.32 times the original. The average gully head retreat rate of newly formed gullies was three times that of the original, up to 0.58 m/a, where the maximum speed reached 2.12 m/a. 3) The drainage area significantly promoted the development of gully heads. A critical topographic index was selected to simulate the retreat rate of gully heads. There was more severe soil erosion under extreme rainfall conditions. The source of newly formed gullies was rapidly traced within three years after the occurrence. Therefore, the appropriate management can be attached to prevent such gullies during this time. This finding can provide an essential reference to estimate the rill erosion on the different types of sloping land in the hilly area of Loess Plateau.