Threshold standard of erosive rainfall under different underlying surface conditions in the Loess Plateau Gully Region of East Gansu, China

Understanding the threshold standard of erosive rainfall is the basis of water erosion prediction. Most previous studies have not fully considered the variability of different underlying surface conditions, and have not effectively combined the thresholds for rainfall amount and rainfall intensity. Based on the rainfall and sediment yield data from 1954 to 2014 for the Nanxiaohegou watershed, a typical small watershed in the Loess Plateau Gully Region of East Gansu in China, the rainfall erosivity deviation coefficient method was applied to analyze threshold standard of erosive rainfall under different land use types and spatial scales. The results of this study can be drawn as follows: ① The average rainfall amount threshold of the underlying surface with soil and water conservation treatments (forestland and grassland) increased by 141% and 116%, respectively, compared to that for bare plot (8.7 mm) and farmland plot (9.7 mm). Thus, vegetation measures can significantly improve the threshold standard of erosive rainfall; ② The erosive rainfall amount threshold of Yangjiagou small watershed was 16.5 mm, which was lower than that for the forestland slope area located in the watershed. In addition, the order of rainfall amount threshold for grassland at different spatial scales was grassland slope scale > grassland total slope scale > grassland watershed scale. These results indicated that the threshold standard of erosive rainfall decreased as spatial scale increased; ③ The best erosive rainfall threshold of each sample was the composite-factor standard: P≥a or I₃₀≥b. For the single-factor standard, it is advisable to apply I₃₀ for farmland and bare land; however, the rainfall amount threshold is considered to be more suitable for the underlying surface with vegetation measures. The results of this study can provide a reference for predicting regional soil erosion in the Loess Plateau Gully Region of East Gansu.