4.2 Effect of land cover on the soil wetting process
In the case of a certain rainfall amount, land-cover type was the key
factor that determined the SM response. Different land-cover patterns
have different vegetation structures, coverage (canopy, herb, litter),
root depths and densities, and soil textures, which influence the soil
wetting process after precipitation (Chen et al., 2007a; Jia et al.,
2017; Li and Shao, 2006; Zhang et al., 2021).
First,
the rainfall-SM average response depth increased by 15.7%, 1.7%, and
26.7% when the crop was transformed to forest, shrub, and grass sites,
respectively (Supplemental Fig. S1). Vegetation recovery altered the
plant coverage and structure radically, resulting in changes in the way
and process of soil acceptance from precipitation (Lozano-Parra et al.,
2015; Wang et al., 2013). Specifically, planted forests, with a higher
canopy and greater litter coverage, intercepted the large rainfall and
weakened the dynamic potential energy of heavy rains in large amounts.
One part of the rainfall is intercepted by the canopy and flows down the
trunk to the soil directly, and the part of rainfall may reach the
litter layer as throughfall. The thick and rough litter hampered the
formation of surface runoff and percolated rainwater infiltration to the
deep soil layer. This vegetation structure promoted an SM response
deeper in planted forests after heavy rains (Fig. 3a). Abandoned grass
improved soil texture, decreased soil bulk density, and increased soil
porosity, which was also beneficial for rainfall-SM infiltration (Sun et
al., 2018). Especially in Fig. 3c, permitting percolation to the 100 cm
depth occurred in heavy rains. Planted shrubs with a high coverage could
not reach the deepest monitoring layer, probably due to SM consumption
by root absorption and vegetation evapotranspiration (Yu et al., 2017).
Additionally, the shrub site showed a shallower average wetting depth
than that of bare land across the 13 rainfall events (Supplemental Fig.
S1). This is because smaller rainfall amounts with short durations have
difficulty penetrating dense canopies and thick litter layers of shrubs
to percolate into subsurface soil (Wang et al., 2013). Compared to
afforestation covers, bare land had a deeper average soil wetting depth
than shrub and crop sites after precipitation. Because no vegetation
coverage results in direct rainfall and surface soil cracks that
increased soil water percolation access together promoted rainwater
infiltration into a deeper layer in bare land. These results suggested
that land cover strongly influenced the rainfall-SM response depth,
while revegetation promoted rainwater percolation to deeper depths
across the profile.
Second, the rainfall-surface SM response lag time (RT) and lasting time
changed significantly due to land-cover change. The results showed that
shrub has the smallest RT and lasting time at the 10-cm depth (Table 3
and Fig. 4). This case was correlated to the initial SM that influenced
the speed of rainwater infiltration (Liu et al., 2019). Due to excessive
soil water consumption by vast evapotranspiration (including soil and
plants) and deeper root water absorption, the shrubland showed the
smallest SM across the entire profile, especially in the surface layer.
The extremely dry situation accelerated soil water permeation, which was
demonstrated by Li et al. (2015) and Sun et al. (2018), who illustrated
that a lower initial soil water content accelerated the topsoil
permeability rate. Thus, planted shrubs have the shortest response and
duration. Compared to shrubs, planted forests and abandoned grass show
longer RT and shorter durations. Due to the existence of a dense canopy
and a thick litter layer, rainfall is blocked to a large extent and
postpones the contact time of rainfall-SM, resulting in a longer
response. Meanwhile, much infiltrated rainwater percolates into deep
soil or counterbalanced with plant consumption, also decreasing the SM
duration at the 10-cm depth. Thus, the effect of afforestation on the
surface SM response time or duration after rainfall was significantly
different. However, with no vegetation coverage, bare land showed a
smaller RT and the longest duration. Because bare land does not have
vegetation shade or litter cover, rainwater directly contacts the
surface soil and decreases the SM feedback time. Therefore, vegetation
recovery delays the surface SM response time but shortens the duration
after precipitation, except for shrubs.
Finally, the change in land-cover type influenced the SM response time
and wetting front velocity (WFV). For example, the crop site required
more time to respond to rainfall, resulting in the longest ART and the
slowest WFV across the entire profile. Vegetation restoration altered
this situation. Vegetation restoration no only shortened the ART by
approximately 30.8%, 33.7%, and 17.4% but also accelerated the WFV by
approximately 77.9%, 36.7%, and 87.0% for planted forest, shrub, and
grass, respectively, over the 1-m depth (Table 4 and Supplemental Fig.
S3). This difference in soil permeability was related to soil texture.
Afforestation improved soil texture, decreased soil bulk density and
increased soil porosity, which facilitated rainwater percolation across
the profile (Sun et al., 2018). Thus, revegetation had the shorter RT
and higher WFV, which benifited for alleviating the soil water deficit
caused by plant water consumption.
In conclusion, vegetation restoration has a significant impact on
ecohydrological processes, including rainfall-SM response depth, RT, and
WFV. Despite afforestation consuming more soil water and causing soil
desiccation to a large extent. However, the rainfall-SM response process
showed that revegetation (mean value of forest, shrub, and grass)
increased the soil wetting depth by 14.7%, shortened the ART by 27.3%,
and accelerated the WFV by 67.2% compared to cropland over the entire
profile. Revegetation contributed to rainwater infiltration and deeper
SM replenishment in the rainy season.