Changes in CI and IR over time
The changes in CI over time for
the HS and W-RS samples are shown in Fig. 4, and the change in IR over
time is shown in Fig. 5. Because of the influence of the ordinate axis
scale, the CI and IR changes do not appear to be significant when\(\theta=\)4.7% (Figs. 4(b) and 5(b)). Thus, we scaled the ordinate of
CI and IR, and present the enlarged plots in Fig.
6.
Figure
4 shows that higher values of ISWC produce a larger CI in both
the HS and W-RS samples. The
difference is that CI exhibits smooth monotonic growth for HS
(Fig. 4(a)), but a more variable
increase for W-RS. The WR effect is very evident in these CI curves,
similar to those measured by
Moret-Fernández et al. (2019),
where
the CI has an inflection point and
demonstrates smooth monotonic growth at the critical point (Fig. 4(b)).
Lichner et al. (2013) defined the relationship between CI and the square
root of time as “hockey-stick-like,” and suggested a procedure for
estimating the WR cessation time. Figure 5 shows that larger values of
ISWC give a larger IR. The IR of the HS sample decreases monotonously
with time (Fig. 5(a)) because of the decreasing contribution of the
capillary (or pressure head)-driven component of the hydraulic gradient
that develops as the wetting front moves further away from the surface
(Lichner et al., 2013). The IR then approaches steady infiltration. In
contrast, the IR of the W-RS sample (Fig. 5(b)) exhibits a gradual
decrease in the initial period (0–20 min), and then presents a
single-peak curve that increases before decreasing.
The larger ISWC values (θ=6.2%
and θ=9.6%) produce an earlier inflection point
(\(\cong\)310 min) than the
smaller ISWC (θ=4.7%, \(\cong\)568 min). For all three ISWCs, the wave
peak of the IR is narrow and the process of reaching the maximum IR is
dramatic, and the attenuation process is slower after the peak. That is,
the IR presents an asymmetric single-peak curve with a left-skewed
distribution.
From Fig. 5(b), we can observe that the stable IR after the peak is
larger than that before the peak. In other words, the water enters the
soil slowly before the inflection point, and then enters more quickly
after this point. This result is in accordance with that reported by
Moret-Fernández et al. (2019). The slower infiltration before the
inflection point could be explained by the hydrophobicity preventing
water from entering the soil, resulting in a low IR. In essence, the
hydraulic gradient is mainly determined by the matrix in unsaturated
flow (Vogelmann et al., 2017), and WR reduces the matric potential.
Hence, the volume of water-conductive pores is lower, promoting a
decrease in IR. Following the wetting of hydrophobic compounds,
the repellency effect disappears
after prolonged contact with water (Filipović et al., 2018), allowing
the IR to increase as the WR
disappears (Alagna et al., 2017).
Another viewpoint is that any initially postponed infiltration increases
after the CA between the water and the soil particles decreases (Bughici
et al., 2016). The mechanisms for
IR increase suggested by Alagna et al. (2017) and Bughici et al. (2016)
are basically the same when stated in terms of CA.
The CI curve is relatively smooth before and after the inflection point
(Fig. 4(b)), and the IR has a significant peak when \(\theta\)=6.2% and\(\theta\)=9.6% (Fig. 5(b)). However,
Fig. 6(a) shows that the CI has
several inflection points and a distinctly mutated inflection point at
580 min when \(\theta\)=4.7%. In this case, the CI exhibits a
multi-stage growth trend. This is consistent with
the results of Filipović et al.
(2018) using HYDRUS (2D/3D) to simulate the hydraulic performance of
W-RS under drought conditions.
Figure 6(b) shows that there are multiple peaks and troughs on either
side of the maximum IR (580 min). This suggests that the infiltration
process of W-RS has unstable infiltration properties when the ISWC value
is small, which may be caused by spatial variability in the WR (Liu et
al., 2019) leading to preferential flow characteristics in WR soils (Rye
et al., 2017). Because of the irregular distribution of the priority
path, the infiltration properties are unstable. Unstable infiltration
may also be caused by the surface pressure head being less than the
water-entry value, which is positive in repellent soils (Wang et al.,
2000). However, taking 580 min as the cutoff when \(\theta\)=4.7%, the
IR gradually increases before and gradually decreases after this point.
From a macro perspective, this is still a single-peak curve.
Wang et al. (1998) suggested that when the air pressure ahead of the
wetting front reaches an air-breaking value, soil air escapes from the
surface, leading to an immediate decrease in the air pressure and an
increase in the infiltration rate. When the air pressure falls below a
certain air-closing value, air escape stops, the infiltration rate
decreases, and the air pressure increases. This cyclic process repeats
itself during the entire infiltration period, possibly explaining the
inflection point in CI (Figs. 4(b) and 6(a)) and significant variability
in IR (Figs. 5(b) and 6(b)), which are inconsistent with the traditional
infiltration theory.
For the W-RS, before the experiment, we adjusted the water head
difference (water-ponding depth)\(h_{0}\) between the Markova bottle and soil surface to 1 cm. After a
period of infiltration, the soil surface was 2 cm higher than the
initial surface as a result of volume expansion (Fig. 14(a)). The soil
water was considered to be within 1 cm positive pressure
infiltration for a short time
after the beginning of the experiment, followed by \(h_{0}\)=1 cm − 2
cm=−1 cm negative pressure with a water-entry value
of \(h_{\text{we}}>0\) (Wang et
al., 2000). Therefore, the water head difference \({}_{h}\) between the
water-ponding depth and the water-entry pressure head satisfied some
pressure criterion, and when\({}_{h}=h_{0}-h_{\text{we}}=-1cm-h_{\text{we}}<0\), unstable
flow occurred (Wang et al., 1998).