4.1 Effect of rainfall on soil moisture dynamics
In the Chinese Loess Plateau (CLP), precipitation is the main source of
soil moisture (SM) (Jia et al., 2017; Su and Shangguan, 2019). Although
the average annual precipitation decreased over nearly half a century on
the CLP (Fu et al., 2017), the season-rainstorm occurrence frequency did
not significantly change with vegetation construction (Tang et al.,
2018). However, under rainstorms, the runoff (Lü et al., 2012) and
sediment (Wang et al., 2016) concentrations decreased significantly
after afforestation, such as the “Grain For Green Project”, radically
altering the rainfall-SM response relationship, which intercepts
rainfall, delays surface runoff, and increases rainwater infiltration
(Kijowska-Strugala et al., 2018). Nevertheless, the characteristics of
the soil water cycle process have changed, and the extent and process of
this change are still unclear.
Our results concluded that the rainfall-SM response exhibited a positive
but not synchronous correlation. The fluctuation of SM occurred after
precipitation, especially in larger amounts of rainfall. The shallow
soil layers were more susceptible to rainfall than deep layers (Fig. 2).
This phenomenon was more obvious under different rainfall patterns with
different rainfall amounts, durations, and intensities in the CLP (Hou
et al., 2018; Jin et al., 2018; Tang et al., 2019). For example, heavy
rains (Group Ⅰ) with a larger amount and intensity had the deepest SM
response depth, at least 70 cm depth (Fig. 3), which was similar to Wang
et al. (2013). Tang et al. (2019) also illustrated that a larger
rainfall amount could promote rainwater percolation into deeper soil.
However, high intensity with a continuous input of rainfall might
surpass the max-rate of soil permeability and limit the soil water
infiltration in the shallow layer (Yan et al., 2021), resulting in the
shortest lasting time of the 10-cm depth and the slowest permeating
velocity of the 100-cm profile (Tables 3 and 4). Compared with heavy
rains, intermediate rains (Group Ⅱ) showed the shallowest response depth
(Fig. 3) but the shortest response time (RT) of the entire profile and
the fastest wetting front velocity (WFV) of the 10-cm SM (Tables 3 and
4). It was indicated that small rains with a high rainfall intensity
could penetrate the dense canopy cover and litter layer to trigger a
surface SM response in the most effective way of all the rainfall
patterns, which was similar to the results of Liu et al. (2020).
Continuous rains (Group Ⅳ) were the most lagging and slowest pattern to
cause an SM feedback. For instance, continuous rains not only lagged in
RT of the entire depth but also showed the smallest WFV in the 1-m
profile across all rainfall patterns. This result was mainly due to a
smaller rainfall amount with a longer duration stretching the rainfall
time and decreasing the average rainfall intensity, which made it
difficult to through the canopy and litter layer, and to store rainwater
in the surface soil layer. There has not been enough water to trigger
the SM response and infiltrate into the deep layer at a larger speed of
the soil wetting front, due to the lack of guidance from
the
gravitational water potential (Mao et al., 2018). Therefore, a high
average rainfall intensity with a smaller rainfall amount promotes a
quick surface SM response, but a larger rainfall amount facilitates
rainwater percolation into deeper soil with a larger WFV.
In addition, regardless of the vegetation pattern, the surface SM (10-cm
depth) responded only when the minimum accumulated rainfall amount (ARA)
surpassed 5 mm. This conclusion is slightly different from Jin et al.
(2018), who demonstrated that a 9 mm ARA was necessary to trigger
surface SM variation. Perhaps the difference in canopy coverage and
litter depth due to vegetation age, which revegetation after 60 years in
Jin et al. (2018). After 20 years of revegetation in this region, a 5 mm
ARA was the threshold needed to replenish soil water, which was consumed
by plant-inducing dried soil layers (Jia et al., 2017; Wang et al.,
2011). It is suggested that the constraint of a minimum ARA exists in
terms of vegetation restoration in semiarid CLP.