3.2 Temporal and spatial evolution law of morphological features of gullies
3.2.1 Number and frequency of gullies
In 2009, 2012, 2018, and 2021, the number of gullies was 1,153, 2,045, 2,413, and 2,312 respectively, and the frequency of gullies is 34.32, 60.86, 71.82, and 68.81 per km² respectively, with a frequency development rate of 2.87 km–2 y–1. Figure 4 shows the spatial distribution evolution of gullies in Zhoutungou watershed from 2009 to 2021, mostly in the hilly southern half of the watershed, which has complex landforms and large slope gradients. From 2009 to 2012, many new gullies formed in the northeast corner and southern half of the watershed, while others disappeared. From 2012 to 2018, new gullies formed in the central south and southeast regions of the watershed, while others disappeared in the northeast corner and southwest regions. From 2018 to 2021, only a few regions formed new gullies, with many disappearing in the northeast corner, central and western regions, and southeast regions. There are three possible reasons for the disappearance of gullies: (1) In the study area, the tail of the gully generally connects with the head of the gully, and headward erosion may engulf the gully; (2) In 2011, the Chinese government implemented a gully land consolidation project on the Loess Plateau, which damaged the slope resulting in the disappearance of some gullies; (3) Returning farmland to forest on the Loess Plateau has resulted in lush vegetation, making it difficult to see some gullies in the remote sensing images.