Morphological characteristics and erosion rate estimation of gully in Hot-Dry Valley of Jinsha River of China

Gully erosion is one of the most important indicators of soil loss in the dry-hot valley of the Jinsha River. Serious land degradation has threatened regional agriculture and food security. It is of great significance for the quantitative evaluation of gully erosion and land protection. In this study, the erosion rate was determined from the morphology evolution and development of the gully. The research area was selected as the gully geomorphic watershed near Jinlei National Soil and Water Conservation Science and Technology Demonstration Zone in Yuanmou Dry-Hot Valley. 35 gullies were selected to extract the gully shape parameters from the remote sensing images of unmanned aerial vehicles (UAV). Morphological characteristics were analyzed to calculate the gully erosion rate combined with high-resolution remote sensing images from 2015 to 2022. The results show that: 1) The straight length, curve length, top width, bottom width, depth, area and volume of gully in the study area were 25.21-180.43 m, 25.73-203.01 m, 10.76-51.76 m, 3.56-26.85 m, 1.22-11.92 m, 257.50-8987.88 m² and 142.64-19479.25 m³, respectively. There was a great variation in the morphological parameters of the gully. The length of the gully was less restricted by slope than other areas during development. 2) The ratio of length to top width of gully (L/TW) ranged from 1.35 to 6.38, with an average value of 2.78. There was no significant gully narrowness. The strong lateral erosion occurred at the same time as retrogressive erosion. The ratio of the bottom width to the top width of the gully (BW/TW) ranged from 0.24 to 0.59. The flushing channels were mainly in the "V" and "V-U" shape. The ratio of top width to depth of gully (TW/D) ranged from 2.56 to 26.29, with an average value of 6.04. The lateral erosion rate was much higher than that of the downward. There was a great difference in the cross section of the gully from gully head to mouth. The tortuosity index of gully (TI) ranged from 1 to 1.13, with an average value of 1.05. There was a small bending degree of the gully. 3) 54.29% of the 35 gullies experienced an increase in length, where the retrogressive erosion rate of the gully was 0.02-1.10 m/a (average 0.34 m/a). 77.14% of the gullies experienced an increase in width, where the lateral erosion rate ranged from 0.07 to 1.10m/a (average 0.36m/a). 80.56% of the gullies experienced an increase in area, and the growth rate of the gully area was 1.64 -105.40m²/a (average 23.44m²/a). The optimal fitting was obtained for the power function of 35 gullies area and volume: V=0.058 4A^1.405, with R²=0.946 1. The volume of the gully was calculated to combine the gully area from the GF-2 remote sensing image. The volume erosion rate of the gully ranged from 1.26 to 339.42 m³/a (average 51.21 m³/a). Both the number and rate of lateral erosion gullies were greater than those of retrogressive erosion gullies. Lateral erosion greatly contributed to the growth of the gully area and volume. The findings can provide a strong reference for the development and evolution of large-scale gullies, in order to quantitatively evaluate the erosion in the Dry-Hot Valley of Jinsha River.