1. Introduction

Soil is fundamental for life on earth, impacting global issues such as food and water security (Koch et al., 2013; McBratney et al., 2014), and is increasingly considered the main contributor to extensive ecosystem services (Dominati et al., 2010). Soil erosion is the greatest threat to soil function at the global scale (Montanarella et al., 2016) and is recognized as the most dominant land degradation process, severely damaging infrastructure, causing land degradation and water pollution in developed regions, and threatening the safety of human production and life in many developing regions (Hao et al., 2020; Mohammed et al., 2020; Pennock, 2019). Ephemeral gully erosion is a linear erosion pattern occurring on steep cultivated slope farmland, formed by the combined action of runoff erosion and human cultivation, and with a connecting role in the slope-gully system (Liu et al., 1988; Poesen et al., 2003; Wang et al., 2003). Ephemeral gully erosion is a major contributor to soil erosion worldwide, adversely affecting water quality in the receiving water courses and impacting aquatic life (Fox et al., 2016; Gholami et al., 2019; Gupta et al., 2019; Anache et al., 2018; Devátý et al., 2019; Eekhout and de Vente, 2022; Sun et al., 2014; Tamene et al., 2020).
Research has focused on measuring ephemeral gully morphological parameters and determining their development at temporal and spatial scales. Some studies have used ground-based methods (e.g. field plots, erosion pins, tape method, volume replacement method) to garner relevant ephemeral gully data (Boardman et al., 2015; Casalí et al., 2006; Evans and Lindsay, 2010; Gómez-Gutiérrez et al. 2014; Kearney et al., 2018; Perroy et al., 2010; Romanescu et al., 2012; Shen et al. , 2021; Wang et al. , 2020a; Wang et al. , 2021a), but the limited data significantly impacts the reliability of the research conclusions for practical application. Further verification is needed to determine whether the conclusions under small-scale conditions can be extrapolated to larger scales. However, large-area field surveys are needed to accurately grasp the spatial distribution characteristics of ephemeral gully morphological features, requiring a large workload with low efficiency. In recent decades, the spatial resolutions, revisiting times, and detail of remote sensing data and digital elevation models (DEM) have significantly increased, providing crucial data support for accurately identifying the spatial position and morphological parameters of ephemeral gullies (Arabameri et al., 2019; Cao et al ., 2020; Dai et al ., 2020; Karydas and Panagos, 2020; King et al ., 2005; Liu et al ., 2022; Vallejo Orti et al., 2019; Yermolayevet al ., 2020). Ephemeral gully volume is an important morphological parameter of ephemeral gullies and an indicator of the contribution of ephemeral gully erosion to sediment yield (Kompia-Zare et al., 2011; Woodward, 1999). Understanding ephemeral gully development over time is important for predicting their future behavior, which can be done directly by measuring volume change (Li et al., 2017). Unfortunately, due to ephemeral gully characteristics and the inherent accuracy limitations of DEM, measuring ephemeral gully volume using DEM data has not been widely used. However, ephemeral gully length is the most easily obtained morphological parameter, with several studies exploring the relationship between ephemeral gully volume (V ) and length (L ) using the power equationV=aLb (Capra et al., 2005; Frankl et al., 2013; Kompani-Zare et al., 2011; Li et al., 2017; Muñoz-Robles et al., 2010; Zhang et al., 2007; Zucca et al., 2006).
Watershed is the basic hydrological response unit, an ideal spatial scale for studying soil and water losses, and the fundamental unit for managing ecological environments. Studies on soil erosion at the watershed scale have focused on estimating soil erosion and sediment (Cao et al. , 2022; Galdies et al. , 2022; Lufira et al. , 2022), assessing erosion susceptibility (Pandey et al. , 2021; Tahouri et al. , 2022; Tesema, 2022; Wang et al. , 2022), and analyzing the factors affecting soil erosion (Aneseyeeet al. , 2020; Choudhury et al. , 2022; Guduru and Jilo, 2022; Zhao et al. , 2022). However, most scholars chose typically representative ephemeral gullies or slopes as research objects to analyze changes in ephemeral gullies (Capra et al., 2009; Karydas and Panagos, 2020; Li et al., 2016; Wang et al. 2021b; Zheng et al., 2006), but typical ephemeral gullies or slopes do not fully display changes in ephemeral gullies at a watershed scale. Therefore, clarifying the temporal and spatial development processes of ephemeral gullies at the watershed scale is valuable for determining soil and water conservation practices and understanding the characteristics of each ephemeral gully and other geomorphologic processes related to ephemeral gully erosion.
The Loess Plateau of China is a typical loess geomorphic region and one of the most serious soil erosion regions in China, if not the world (Fuet al. , 2011). Ephemeral gully erosion on the Loess Plateau accounts for more than 70% of its ravines, with erosion accounting for 26.60–59.20% of total slope erosion (Cheng et al., 2007; Zheng et al., 2006). Ephemeral gully erosion control has been underway on the Loess Plateau since 1999 when the Chinese government implemented ‘Grain for Green Project’ (Fu et al., 2017; Ran et al., 2013; Wang et al., 2014), with some areas still requiring attention. This study used 0.50 m resolution remote sensing images integrated with a deep learning image semantic segmentation model (deep learning can scientifically build high-level features from raw datasets, deliver forecasting results, and perform better than machine learning in various research areas) to identify ephemeral gullies and extract morphological features at the watershed scale, analyze the temporal and spatial distribution and evolution law of ephemeral gullies, and clarify the development processes and rate of ephemeral gully erosion at the watershed scale in the hilly and gully region of the Loess Plateau. This research will help select the most effective countermeasures to prevent and control soil erosion and provide more effective measures to protect limited land resources and ecological environments.