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.