Figure 7. Changes in the number of gullies before and after the project returning farmland to forest in the Zhoutungou watershed
Numerous studies have analyzed the temporal and spatial distribution of gullies on the Loess Plateau (Chen, 1984; Guo and Wang, 2019; Kanget al. , 2016; Zheng et al. , 2016; Zhong et al. , 2022). This study found that the gully number increased by 1,159 from 2009 to 2021, with an average growth rate of 97 per year, mostly in the hilly southern part of the watershed, which has complex landforms and large slope gradients. From 2009 to 2021, gully lengths ranged from 5 to 230 m (average 48.39 m), of which 55% ranged from 25 to 40 m, with an average growth rate of 1.66 m y–1. However, other studies reported gully lengths ranging from 7.71 to 237 m (average 56.5 m), of which 60% ranged from 30 to 60 m (Jiang et al., 1999; Li, 2011; Qin et al., 2010; Zhang et al. 2017; Zhong et al. 2022). The shorter gully lengths in this study may be related to the more recent conversion of farmland to forest, inhibiting the development of gullies.
In terms of length, this study found that more than 99% of the gullies are 0.5–1.5 m wide, with an average growth rate of 0.04 m y–1. Similarly, Liu et al. (1988, 2018) reported gully widths in loess hilly and gully areas of around 0.3–0.5 m, sometimes 1–2 m. The total area of gullies ranged from 0.0566–0.1111 km², accounting for 0.17–0.33% of the total watershed area. However, Jiang et al. (1999) reported a much higher value for the total Zhoutungou watershed area in 1999 (4.0 km² or 11.89% vs. 0.0566 km2 or 0.17%).
To sum up, while some research results are based on the temporal and spatial distribution characteristics of gullies on the slope of the Loess Plateau, and the project returning farmland to forest has controlled the gully development, the frequent occurrence of uncertain extreme rainfall events and poor consolidation of the results of returning farmland to forest are the main reasons for the increased gully erosion on the Loess Plateau in recent years. Soil erosion control and innovative erosion control strategies need further investigation.