3. Results and discussion
3.1. Soil erosion rate
Under a rainfall intensity of 90 mm/h, the soil erosion variation range
rate for the four soil types was 1.8-35.4 g min-1m-2 (SL), 27.1-135.4 g min-1m-2 (LL), 16.5-76.0 g min-1m-2 (ML) and 105.2-467.1 g min-1m-2 (HL), having the order of SL < LL ≈ ML
< HL (Fig. 2). When rainfall intensity was 120 mm/h, the rate
of soil erosion for the four soil types increased by 11.9 times (SL),
1.6 times (LL), 2.3 times (ML) and 1.4 times (HL). Erosion rate at SL
was the most sensitive to an increase in rainfall intensity, and the
overall order of soil erosion rate was ML ≈ LL < SL
< HL. These results indicate that rainfall intensity plays a
major role in soil erosion. Soil erosion rate with slope recorded an
initial increase for SL and HL before decreasing as the angle of slope
increased; maximum erosion rates were recorded at 15° and 20°,
respectively. The soil erosion rates for LL and ML had no regularity
with an increase in slope angle.
In order to further analyze causes for different types of soil erosion,
differences in erosion rate before rills were formed were analyzed.
Results indicate that, although no significant differences in SL, LL or
ML were recorded, a significant difference between SL, LL, ML and HL was
evident (Fig. 3). As shown in Fig. 2, significant differences among the
four soils during the whole soil erosion process were also recorded.
Comparison of these results indicate that differences of SL, LL and ML
soil erosion were mainly caused by differences in rill erosion. These
results also indicated that an increase of soil erosion rate caused by
different types of soil rill erosion was not obvious when rainfall
intensity was 90 mm/h. When rainfall intensity was 120 mm/h, SL rill
erosion caused a sharp increase in the soil erosion rate; LL rill
erosion caused a strong increase in the soil erosion rate; and ML rill
erosion caused a slight increase in the soil erosion rate. An increase
in the soil erosion rate caused by HL rill erosion was not obvious.
3.2. Morphological development and occurrence of
rills
3.2.1. Occurrence of rills
Runoff characteristics and rill generation time (Table 2) indicate that,
when rainfall intensity was 90 mm/h, rills were evident in all slopes
under the HL soil type. For SL, LL and ML, rills were only recorded with
slope angles of 15°, 20° and 25°, respectively. The occurrence time of
rills gradually decreased with a decrease in particle size, a finding
that is consistent with the variation law of runoff generation time
(Table 2). Under a rainfall intensity of 120 mm/h, rills appeared in HL
and SL at all four slopes; LL recorded an absence of rills at 15° and
25°, and ML had no rills at 10°. Rill frequency results indicate that HL
was the most prone to the formation of rills, and rills occurred more
frequently at SL, LL and ML under heavy rain, with SL being the most
sensitive to heavy rain. Under different rainfall intensities and
slopes, a decrease in loess particle size resulted in a gradual decrease
in runoff generation time, and average runoff rates before rill
formation gradually increased (Table 1 and Table 2). However, the
uniformity of soil particle size composition and the size of aggregates
affects topsoil stability (Neyshabouri et al., 2011; Madenoglu et al.,
2020). In general, the stability of topsoil decreases with a decrease in
uniformity of soil particle size composition and an increase in
aggregate size (Neyshabouri et al., 2011). Our results indicated that
particle size composition of ML and HL was uniform (Table 1), and that
particle size of aggregates was larger. Although these characteristics
were conducive to runoff stripping and scouring, organic matter content
in ML was higher than in the other soil types, thus enhancing surface
soil stability. Before rill formation, HL recorded rates of runoff and
soil erosion. A large volume of sediment was stripped and transported,
resulting in the formation of an uneven slope, thereby creating
conditions for concentrated flow. HL was also recorded to have the
highest clay content of the soil types, thus being more readily stripped
and scoured, thus being the main reason why this soil type was prone to
rill formation.
As previously recorded (He et al. 2017), rills are mainly caused by
heavy rainfall intensity; consistent with our results, rills typically
increase with an increase in rainfall intensity (Berger et al., 2010).
Rainfall intensity affects soil erosion through raindrop strike and
runoff (Vaezi et al., 2018). Large rainfall intensity increases
instantaneous runoff, and an increase in raindrop strike increases
runoff disorder, which increases runoff erosivity, thus increasing the
probability of rill generation. By comparing runoff rate and runoff
velocity under different types of soil before rill formation indicated
that rill formation is more sensitive to an increase in runoff rate
caused by an increase in rainfall intensity. Under a rainfall intensity
of 90 mm/h, SL runoff was low, and runoff was difficult to converge,
therefore not being conducive to the generation of rills. Under a
rainfall intensity of 120 mm/h, the increase in raindrop splash erosion
and runoff promoted the removal and transportation of sediment,
resulting in the formation of an uneven slope. As SL contained the
largest sand content, a loose structure and the lowest content of
organic matter, this soil type had poor structural stability. Once
uneven points are present in a soil, it will quickly collapse, promoting
the generation of rills. Heavy rainfall intensity also increased
stripping and transportation of sediment under LL and SM soil types.
However, due to LL and ML having higher clay and organic matter
contents, soil structural stability of these soil types increased,
resulting in a reduction of soil collapse. At the same time, because
silt and sand contents were still large, erosion and scouring were not
easily undertaken, thus rills did not readily form under LL and ML
soils. Even when rills did form under these soil types, formation took a
long time to occur.
Although SL, LL and ML rills did not record an obvious change with slope
gradient, rills still mainly occurred on slopes with a greater gradient,
indicating that slope gradient was an important factor affecting rill
development (Mancilla et al., 2005). Slope affects rill generation by
affecting rainfall, the runoff dynamic gradient and soil stability
(Bagarello et al., 2010; Langhans et al., 2014; Jerzy et al., 2016). In
general, the hydraulic gradient of runoff from a steep slope is large.
This increases the velocity of slope flow and increases the shear force
of slope runoff; at the same time, the instability of a steep soil slope
increases, and the component force along the slope surface increases.
Sediment is therefore removed by runoff, thus being conducive to the
occurrence of rills (Komatsu et al, 2011; Ries et al., 2014; Zhang et
al., 2016). However, under the influence of reduced the area of land
experiencing rainfall, the impact of slope on runoff has a critical
value. The sensitivity of different types of soil to a reduction of
rainfall differs, and the occurrence time and probability of rills vary
with slope angle.
3.2.2. Morphological development of
rills
Different indicators are currently used to quantify the development
process of rills from the aspects of rill cross-section morphology, rill
network characteristics and connectivity. In this study, cumulative rill
length (CDL), average rill width (WA), average rill depth (Ha), rill
density (DS), rill merging node (J) and rill number (N) were selected
from these aspects to quantify rill morphology variation with rainfall.
These variables were also used to clarify differences in rill
development processes for the different types of soil, such as headward
erosion, rill wall collapse and rill bottom undercutting.
Results for change process of CDL with rainfall (Fig. 4) indicated that
SL, LL and ML had an approximate linear increase with an increase in
rainfall; the increment of increase, however, was small. CDL of HL
recorded an initial rapid increase as rainfall increased; after this the
rate of increase declined. The increment for HL was large. At the end of
the rainfall experiment, CDL range for SL, LL and ML was 1.5-4.8 m, and
there was no obvious sequence relationship among these soils. CDL range
for HL was 8.2-41.7 m. CDL for SL increased with an increase in rainfall
intensity, and CDL changes for LL, ML and HL were almost not affected by
rainfall intensity. CDL for SL initially increased before decreasing
with an increase in slope, reaching a maximum when the angle of slope
was 15°. CDL for LL and ML did not record a change with an increase in
slope angle; CDL results for HL recorded an increase as the angle of
slope increased.
Results for Wa change process with rainfall (Fig. 5) recorded an
increase with an increase in rainfall duration. Results for SL and LL
recorded an initial rapid increase before the rate of increase declined;
ML recorded a rapid increase at 20° and 25° under a rainfall intensity
of 120 mm/h, having an initial rapid increase and then increasing slowly
under other conditions. At the end of the rainfall experiment, Wa for
the different soil types was: HL < ML ≈ LL < SL, in
which Wa of SL ranged from 12.6 - 24.2 cm, ML and LL ranged from 3.4 -
15.0 cm, and HL ranged from 1.6 - 2.5 cm. The influence of rainfall
intensity on Wa of SL and HL was not obvious; Wa of LL and ML increased
with an increase in rainfall intensity. Wa recorded a decrease for SL
with an increase slope angle; LL and ML recorded an increase in Wa with
an increase in slope angle; and HL recorded no change.
Ha recorded a general trend of increase with rainfall (Fig. 6). Ha
results for SL, LL and HL all recorded a rapid initial increase before
the rate of increase declined; Ha for ML recorded an initial rapid
increase under a rainfall intensity of 120 mm/h. At the end of the
rainfall experiment, no obvious sequence relationship was identified
between Ha under all soil types in the range of 2.5-10.7 cm. The
influence of rainfall intensity on Ha under SL, ML and HL is therefore
not obvious. Ha under LL recorded an increase with rainfall intensity.
Ha under SL recorded an initial decrease before increasing as the angle
of slope increased, with the maximum value occurring when the angle of
slope was 10°. Ha of LL and HL increased with an increase in slope
angle. Under a rainfall intensity of 120 mm/h, Ha under ML decreased
with an increase in slope angle.
The change process of DS with rainfall (Fig. 7) recorded an increase as
rainfall duration and rainfall intensity increased for all soil types.
However, under a rainfall intensity of 120 mm/h, the increasing trend of
DS under different types of soil gradually declined. At the end of the
rainfall experiment, DS for the different soil types was in the order
of: ML < SL < LL < HL. DS ranged from 0.2
- 2.2 m/m2 for SL, LL and ML, and from 3.2 - 7.6
m/m2 for HL. DS of SL initially increased before
decreasing as the angle of slope increased, reaching a maximum at 20°.
DS under LL and HL increased as the angle of slope increased, and DS
under ML was irregular.
Results for the change process of J with rainfall (Fig. 8) indicated
that J under SL easily occurred under a rainfall intensity of 120 mm/h
and a slope angle ranging from 10 ° to 20°. Results indicated that J did
not easily form under LL and ML soil types; J also easily formed under
HL. When rainfall intensity was 90 mm/h, J under HL rapidly increased
with an increase in rainfall duration. When rainfall intensity was 120
mm/h, J recorded a rapid initial increase before increasing slowly with
rainfall duration under SL and HL soil types. At the end of the rainfall
experiment, the variation range of SL and HL was 2-6 and 5-24,
respectively. Although J recorded an increase with an increase in slope
intensity under SL and HL, it was not readily apparent at 25° under SL.
Change processes of N under all soil types recorded an increase with
rainfall duration (Fig. 9), being characterized by an initial rapid
increase which then declined. HL recorded the most N, ranging from 9 to
40. There was no obvious order relationship among SL, LL and ML, with N
ranging from 1 to 7. Results for rainfall intensity recorded N to
increase under all soil types. With an increase in slope angle, N under
SL initially increased before decreasing, peaking at
20o. N under LL decreased as slope angle increased. N
under ML was predominantly not affected by slope, and N under HL
recorded an increase as the angle of slope increased.
Our results indicate that the development process of rill morphological
parameters under different soil types are similar, recording rapid
increases under a light rain intensity and a gentle slope, however the
level of increment was small. Under a heavy rain intensity and a steep
slope, rill morphological parameters initially rapidly increased before
the rate of increase declined; the level of increment of rapid increase
was larger. In the early stage of rill development, the converging
effect of rill development on runoff gradually increased, and converging
runoff further promoted the rapid development of rills. With the
continuous development of rills, the converging effect of rills on
runoff gradually weakened, decreasing rill development. Our results are
consistent to those of Wu et al. (2018) who examined feedback coupling
effects between rill morphology and rill erosion. Our results also
indicated that morphological characteristics of rill development in
different soil types differ. The development of rills under SL was
mainly due to an increase of Wa and J; rill development under HL was
mainly due to an increase of CDL, J and N. The index parameters of LL
and ML rill development were between SL and HL. At the end of the
rainfall experiment, Ha had no obvious order relationship, however the
speed of increase differed during the rainfall events. The main reason
for this phenomenon is that rill widening is mainly caused by collapse
of the rill wall and merging of adjacent rills under gravity (Mancilla
et al., 2005; Shen et al., 2015). An increase in rill length is mainly
caused by an increase in the number of rills and rill head advance under
traceable erosion (Schneider et al., 2013; Wu et al 2018). An increase
in rill depth is mainly caused by further retrogressive erosion of rills
due to undercutting, caused by runoff shear dispersion and the
reemergence of rill head undercutting in rills (Gómez et al., 2003;
Zhang et al., 2017; Zhao et al., 2018). A higher level of soil cohesion,
due to higher levels of clay and organic matter content, make it easier
for soil to form a mass structure, especially as clay content in a soil
can significantly enhance the anti-dispersion ability of the wet soil
layer. Soil structure stability is the important factor here. Therefore,
Wa of HL slopes was smaller, being less than 5 cm. Under the condition
that rills easily occur, stripping erosion due to runoff further
intensifies the headward erosion of rills. SL soil only had a clay
content of 12.1% and the lowest organic matter content of the four
soils, resulting in this soil to have poor structural stability. In
addition, the sand content of SL peaked at 68.5%, resulting in the soil
to be loose and porous. Once a rill formed in SL, the rill wall and
gully head readily collapsed under runoff effects, thus aggravating
slope erosion. Due to differences in rill development mode between SL
and HL, Ha in SL was mainly developed by further retrogressive erosion
of the rill head after rill collapse, characterized by a phased
increase; Ha in HL was mainly developed by rill bottom cutting whilst
the rill length increased, characterized by a gradual increase. At the
same time, due to differences in rill development mode between SL and
HL, the negative effect of SL rill development on runoff convergence
slowly weakened, and the negative effect of HL rill development on
runoff convergence weakened faster. Therefore, after rill formation, SL
erosion is more severe and lasts longer.
Results from this analysis indicate that rainfall intensity has a
positive effect on rill morphological parameters for all soils, and
sensitivity of rill parameters to rainfall intensity was: N
> DS > J > Wa > CDL
≈ Ha. The angle of slope had both positive and negative effects on SL,
LL and ML rill morphological parameters, with only positive effects on
HL. The sensitivity of rill parameters to slope response had the order
of: Ha > Wa ≈ J > DS ≈ N > CDL.
3.3. Correlation between influencing factors and relevant
parameters of erosion
processes
In order to examine the influence of influencing factors on slope
erosion, Pearson correlation analysis was used on influencing factors,
runoff and sediment parameters, and rill shape parameters (Table 3 and
Table 4). Results indicate that clay, silt and MWD were significantly
positively correlated with the amount of erosion and rill erosion, and
sand and OM was significantly negatively correlated with the amount of
erosion and rill erosion (Table 3). The correlation of soil
characteristic factors with rill morphological parameters indicated that
clay and silt are significantly positive correlated with CDL, DS, J and
N. Significant negative correlations were recorded between clay and Wa,
and a significant negative correlation was recorded between silt and Wa,
Ha; a significant positive correlation between sand and Wa, Ha was
recorded, and a significant negative correlation was recorded between
sand and CDL, DS, J and N. A significant positive correlation was found
between OM and MWD and CDL, DS, J and N. finally, a significant negative
correlation existed between OM, MWD and Wa, Ha. These findings correlate
the rationality of our previous analysis.
As soil characteristics also affect slope roughness after erosion, they
also affect flow velocity between interrills and rill, thereby having an
important indirect impact on the development of erosion and rill
morphology. As shown in Table 4, clay, silt and MWD were significantly
positively correlated with Q and V, and significantly negatively
correlated with T1 and VL. Sand was significantly positively correlated
with T1 and VL and significantly negatively correlated with Q and V. OM
was positively correlated with T1 and V, and negatively correlated with
Q. Wa and Ha were negatively correlated with V, CDL, DS and J; N was
negatively correlated with VL and positively correlated with Wa.
3.4. A multi-type soil erosion prediction model based on
rill
development
Correlation analysis shows that soil characteristic factors seriously
affect slope erosion, thus the establishment of a quantitative index of
soil characteristics in this study took erosion amount as the objective
function. Soil characteristic factors with a significant correlation
with erosion amount were selected to establish a regression
relationship, thereby forming a comprehensive quantitative parameter
(C). By using multiple linear regression in SPSS, the silt factor was
eliminated due to collinearity, and the equation was established as:
C=4.220 clay+0.392 sand+49.986 MWD-10.55 OM R2 =0.814
(1)
This analysis indicates that the change of erosion amount has a
synchronous effect with rill morphology evolution. In this study, we
adopted the same method as that used by Zhang et al. (2019),
constructing the rill morphology expression (G) as:
G=6.271 CAL+1.108 Wa+2.100 DS+0.862 N R2 =0.926 (2)
Based on the constructed soil characteristic factor C and rill form G,
combined with the main factors of rainfall, rainfall intensity and slope
that affect soil erosion, the prediction model of slope erosion in the
loess area was established as:
R2=0.919 (3)
where, D is the erosion module of individual rainfall under a bare slope
(kg m-2); P is rainfall (mm); I is
rainfall intensity (mm/h); and S is slope (°). The model was
validated using 78 independent data sets (Fig. 10), and
R2 between the measured value and the simulated value
was 0.919, indicating that the equation could suitably predict slope
erosion.