4.3 Ephemeral gully erosion rate
The world is under serious threat of soil erosion. The ephemeral gully erosion rate in Europe and America is 90–6,490 t km–2 y–1 (Capra et al., 2009; Li et al., 2016; Maetens et al., 2012; Panagos et al., 2015; Valcárcel et al., 2003). On the Loess Plateau, the ephemeral gully erosion rate without vegetation coverage is about 1,000–12,000 t km–2 y–1, and with vegetation coverage is 215 t km–2 y–1 in the Loess Plateau (Zhang et al., 2017; Zheng, 2006). This study estimated an ephemeral gully erosion rate of 22.12 m3km–2 y–1 or 28.75 t km–2 y–1 (average soil bulk density is 1.3 g cm–3) based on high-resolution remote sensing images in the watershed from 2009 to 2021, considerably lower than previous results but attributed to the improved vegetation coverage in the watershed since the Chinese government implemented the ‘Grain for Green Project’ in 1999.
The increased vegetation coverage and biomass results in a dense vegetation canopy that can intercept and redistribute rainwater and reduce the kinetic energy of raindrops, and the grassland cover and plant litter can reduce runoff erosion by avoiding the direct impact of raindrops on soil, increasing soil infiltration capacity and runoff resistance and decreasing runoff velocity (Hayas et al., 2017; Mukai, 2017; Sun et al., 2016; Vanmaercke et al., 2016). In addition, the well-developed root systems have great physical and chemical binding effects for combining soil particles and soil aggregates to improve soil cohesion and the shear strength and tensile strength of the soil–root matrix, significantly enhancing soil resistance to erosion (Allen et al., 2018; Barthès and Roose, 2002; De Baets and Poesen, 2015; Guo et al., 2019; Khan and Lateh, 2015; Wang et al., 2021c; Zegeye et al., 2018; Zhao et al., 2013). In summary, vegetation restoration has improved soil erosion conditions and enhanced regional water and soil conservation benefits.