1. Introduction
China’s Loess Plateau is the most typical loess geomorphic region in the
world and one of the most serious soil erosion regions in China and
perhaps the world (Fu et al. , 2011; Zhong et al. , 2022).
Gully erosion is an important type of soil erosion on the Loess Plateau,
with its distribution area accounting for more than 70% of the ravines
and its erosion accounting for 26.6–59.2% of the total slope erosion
(Zheng et al. , 2006). Soil erosion control on the Loess Plateau
started in 1999 when the Chinese government implemented a project
returning farmland to forest (Huang et al. , 2020; Liu et
al. , 2019); however, gully erosion still needs attention in some areas.
Watershed is the basic unit of hydrological response and an ideal
spatial scale for studying soil and water losses. Clarifying the
temporal and spatial evolution of gullies at the watershed scale is
important for optimizing soil and water conservation measures on the
Loess Plateau.
Gully erosion is a linear erosion pattern occurring on steep slope
cultivated land, formed by the combined action of runoff erosion and
human cultivation, and plays a connecting role in the slope gully
erosion system (Liu et al. , 1988; Poesen et al. , 2003;
Wang et al. , 2003). At present, studies on gully erosion have
focused on critical topographical conditions (Daggupati et al. ,
2014; Feng, 2022; Maugnard et al. , 2014; Torri and Poesen, 2014),
factors influencing gully formation (Feng, 2022; Gong et al. ,
2011; Xu, 2018), mechanical processes and control measures of gully
development (Guo, 2019; Xiao, 2017), flow dynamics and sediment
transport (Ban et al. , 2020; Kang et al. , 2021a; Xuet al. , 2021), and gully erosion models (Douglas-Mankin et
al. , 2020; Guo et al. , 2019; Luquin et al. , 2021; Tekwaet al. , 2021). However, large-area field surveys are needed to
accurately grasp the morphological features and spatial distribution
characteristics of widely and densely distributed gullies, requiring a
large workload and low efficiency. Therefore, some studies have adopted
indoor model tests and field slope unit positioning tests to garner
relevant gully data (Shen et al. , 2021; Wang et al. ,
2020b, 2021), but the limited data significantly impacts the reliability
of the research conclusions for practical application. In addition,
further verification is needed to determine whether the conclusions
obtained under small-scale conditions can be extrapolated to larger
scales.
The recent rapid development of remote sensing technology has provided
high-resolution remote sensing images and data for gully surveys on a
large scale. Some studies have applied remote sensing images for gully
recognition (Cao et al. , 2020; Dai et al. , 2020; Liuet al. , 2021; Yu et al. , 2018), determining temporal and
spatial variation of gullies (Karydas and Panagos, 2020; King et
al. , 2005; Li et al. , 2007; Platoncheva et al. , 2020; Yanet al. , 2005, 2006, 2010; Yermolayev et al. , 2020; Zhonget al. , 2022) and gully erosion sensitivity analyses (Amiriet al. , 2019; Arabameri et al. , 2020; Busch et al. ,
2021; Garosi et al. , 2019). On the Loess Plateau, Zhao et
al. (2011) analyzed the correlation between land use, slope, and gully
distribution using SPOT images, reporting significant differences in
gully density among different land use types (grassland >
forest land > hilly dry land). Zhang et al. (2017)
extracted the lengths of 245 gullies using two QuickBird images,
quantified the relationship between gully length and eroded volume, and
assessed the erosion rate over six years. Qin (2009) and Qin et
al. (2010) analyzed the topographic characteristic parameters and
distribution law of gully erosion using a QuickBird image and digital
elevation model (DEM), revealing that the morphological features of
gullies are determined mainly by slope gradient, slope length, uphill
length and confluence area in hilly and gully regions on the Loess
Plateau. Wang (2020) found high gully development and large overall
erosion potential in the south and north of Dongzhiyuan on the Loess
Plateau and low gully development in the central region, where gully
erosion is mostly caused by human activities. Liu et al. (2014)
and Liu (2012) used 137Cs tracer technology to study
the spatial differentiation of soil erosion of typical hilly slopes in
hilly and gully areas of the Loess Plateau, revealing differences in
hilly slope erosion in different slope directions and an average erosion
rates ranked north slope > southwest slope >
northeast slope > west slope > northwest slope
> south slope > southeast slope >
east slope. Jiang et al. (1999) analyzed the distribution law of
gullies in the Zhoutungou watershed in the hilly and gully region of the
Loess Plateau using aerial images. They found that gullies develop
mainly on slope farmland, with gullies accounting for 31.3% of the
hillslope area in the watershed. Li (2011) found that the gully density
in the loess area of northern Shaanxi, China changes into sunlit slope
< semi-sunlit slope < sunless slope <
semi-sunless slope under the same slope gradient. Tian et al.(2013) investigated the spatial differentiation characteristics of
gullies on the Loess Plateau using 5 m resolution DEM; the results
revealed clear spatial differentiation of gully density, decreasing from
south to north, with gully density peaking in the Suide-Mizhi area of
northern Shaanxi. The above studies were based mainly on visual
interpretation and used independent slope elements in the basin as the
research object. However, it is not possible to clarify the temporal and
spatial distribution and evolution law of gullies at the basin scale.
Few studies have identified gullies by combining deep learning and
remote sensing images or investigated changes in morphological features
before and after returning farmland to forests. Thus, this study used
0.5 m resolution remote sensing images integrated with a deep learning
image semantic segmentation model to identify gullies and extract
morphological features at the watershed scale, analyze the temporal and
spatial distribution of gullies, and clarify the temporal and spatial
evolution law of gullies in typical small watersheds in the hilly and
gully region of the Loess Plateau to provide a theoretical and
scientific basis for watershed-scale gully erosion control and
ecological environment protection on the Loess Plateau.