1. INTRODUCTION
NOTE: What does the response dynamic of SM after rainfall look like under different land-cover types? What content and extent of the soil water percolation process are changed after revegetation? And which afforestation pattern can make the most effective use of rainfall to improve the water conservation function of vegetation? There is still no consensus and definite answer to these questions.
Climate drought and water deficiency are common problems in arid and semiarid regions around the world. Under the background of global warming (Williams et al., 2020), the dryland area will continue to expand (Feng and Fu, 2013; Lickley and Solomon, 2018), from 40% (Huang et al., 2016) of the global terrestrial ecosystem at present to 47% by the end of the 21st century (Koutroulis, 2019). Although water resources are scarce in arid and semiarid regions, soil losses in these regions are extremely serious (Fu et al., 2017; Mamedov and Levy, 2019; Wang et al., 2020), and this pattern is mainly related to the lack of vegetation coverage (Zhao et al., 2013), loose soil and easy erosion (Fu et al., 2017), as well as irregular but high-intensity rainfall in such areas (Mamedov and Levy, 2019). Therefore, drought and water and soil loss have become urgent contradictions in arid and semiarid regions of the world.
To solve this prominent problem, many countries and regions have adopted a close to natural solution, namely, planting trees (Zheng et al., 2016). Afforestation is considered to be the most economical and convenient way to restore the ecological environment and reduce water loss and soil erosion, so the practice has been widely recognized and accepted in the world (Bryan et al., 2018; Chirino et al., 2006; Liu et al., 2018; Wu et al., 2021). Since 1990, the area of artificial forest has increased by more than 105 million hectares, accounting for 7% of the global forest area (Birdsey., 2015). Revegetation has a significant impact on the hydrological process of an ecosystem by influencing evapotranspiration, water infiltration, runoff generation, soil erosion, and solute transportation (Ding et al., 2021; Su and Shangguan, 2019; Zhu et al., 2021), thus altering the terrestrial water recycling process.
The relationship between rainfall and SM has always been a core issue in the study of soil hydrological processes. In recent years, many studies have been conducted in this research field. For example, Rohit et al. (2011) assessed the impact of altered rainfall on soil-moisture dynamics in three annual grassland vegetation types. He et al. (2012) qualitatively described the influence of precipitation on the soil moisture of the rainy season in northwestern China’s Qilian Mountains. Su et al. (2019) and Li et al. (2021b) used meta-analysis to illustrate that the relationship of rainfall and SM changed by vegetation construction and afforestation led to a decline in SM on the Loess Plateau of China. Despite efforts to characterize the spatiotemporal variations in SM related to rainfall, but SM dynamics and infiltration processes cannot be determined after precipitation. In subsequent studies, Pan et al. (2019), He et al. (2020) and Mayerhofer et al. (2017) analyzed soil water infiltration, redistribution and runoff through laboratory or field experiments using simulated rainfall. Many studies have adopted continuous monitoring equipment to analyze SM dynamics after rainfall under different vegetation patterns but have reached different conclusions (Jin et al., 2018; Wang et al., 2012; Wang et al., 2013; Yu et al., 2015). For instance, Wang et al. (2013) suggested that the SM response to rainfall leads to the smallest accumulated infiltration and largest surface runoff occurring in grasslands. Jin et al. (2018) showed that forestland has a faster SM response time and deeper response depth than grassland but the smallest soil water storage. What does the response dynamic of SM after rainfall look like under different land-cover types? What content and extent of the soil water percolation process are changed after revegetation? And which afforestation pattern can make the most effective use of rainfall to improve the water conservation function of vegetation? There is still no consensus and definite answer to these questions.
As a typical arid and semiarid region in the world, the Chinese Loess Plateau (CLP) has suffered extremely serious soil erosion and water and soil loss for a long time (Chen et al., 2007b; Fu et al., 2017). To protect soil and water and curb ecological deterioration, the Chinese government implemented the ‘Grain For Green’ project at the end of the 20th century. Vegetation reconstruction significantly changed the land-cover type and vegetation structure, reshaped the mutual-feeding relationship and cycling process between the soil-vegetation-atmosphere continuum (SPAC) (Deng and Shangguan, 2017; Jia and Shao, 2014), and fundamentally contained water and soil loss and restored the ecological environment on the CLP (Fu et al., 2017). For instance, Zhao et al. (2017) and Fu et al. (2017) found that 20 years of revegetation reduced the runoff and sediment load of the Yellow River by 24.8% and 57%, respectively, which controlled soil erosion to a great extent. Moreover, many studies have confirmed that the conversion of farmland to trees or grasses enhances vegetation coverage and rainfall interception (Liu et al., 2020; Murray, 2014), improves soil texture (Li et al., 2006), and increases soil porosity, thus greatly promoting rainfall infiltration and soil water storage (Kijowska-Strugala et al., 2018; Sun et al., 2018). However, artificial vegetation consumes more soil water due to the larger evapotranspiration (Jia et al., 2017), which intensifies soil desiccation and even forms a dry soil layer (Wu et al., 2021), thus limiting plant growth and resulting in advanced senescence and degradation of artificial vegetation, such as the ‘little old man tree’ (Wang et al., 2010b). Consequently, the soil water deficit is one of the most important problems affecting the survival and sustainability of artificial afforestation (Li et al., 2021a). Improving the conversion efficiency of rainfall is a critical path to solving the soil water deficit problem. Therefore, it is particularly important to study the rainfall-soil moisture (SM) response process and its effect on rainwater transformation and utilization.
Based on the extensive existence of artificial vegetation and the need for understand soil water infiltration processes, this paper aimed to reveal the rainfall-SM response process and mechanism, and evaluate the effect of soil water infiltration and replenishment by different rainfall or vegetation land-cover types to clarified the optimal revegetation type on the semiarid region. To do this, we monitored the 1-h SM at five depths down to the 1 m depth (10, 30, 50, 70, and 100 cm) in typical land covers (forest, shrub, grass, crop, and bare land) over the growing season of 2019. We hypothesized that 20 years of revegetation has changed soil water replenishment patterns, promoted rainwater use efficiency and enhanced soil water storage in the growing season. The results of this study are expected to shed insight into profile soil water infiltration processes related to rainfall after vegetation restoration on the CLP and to provide design and optimization solutions for vegetation restoration in similar areas of arid and semiarid regions around the world.