4.1. Effects of slope gradient and vegetation cover on runoff
In a field-based runoff scouring
experiment, we found that variation of total runoff under different
vegetation covers are divided into three stages (slow at first in 0 to 5
mins, then increased rapidly in 5 to 10 min, and then leveled off in10
to 30 mins). The complicated phenomenon is due to the overall effects of
several factors including scouring intensity, surface conditions
(roughness, sealing layer on topsoil), vegetation covers,
overland flow continuity, soil
particles and infiltration capacity etc. (Lavee and Poesen, 1991; Cerdà,
2001; Chen et al., 2013). These factors have different effects on the
runoff. For example, the scouring intensity affected
sealing layer formation (surface
conditions) which can significantly reduce infiltration rates (Assouline
and Ben-Hur, 2006). Meanwhile, soil infiltration rates is constantly
changing. It is also affected by many factors including soil moisture,
soil texture, surface roughness, and scouring characteristics. During
the process of runoff generation, all these factors are interactive and
the overall effects of them are far from perfectly understood (Zhang et
al., 2018). In this study, runoff is unstable and changes slowly at
first in 0 to 5 mins due to the effect of surface roughness and overland
flow continuity. With scouring time increasing, runoff increased rapidly
in 5 to 10 min is affected by sealing layer formation. Then the runoff
became leveling off in the later scouring period when spatial
heterogeneity of infiltration is basically stable.
In addition, when slope gradient was less than 15°, the difference of
mean runoff and total runoff was obvious under different vegetation
covers, while when slope was steeper than 15°, the difference was less
clear. Variation in runoff that occurred on slopes more than 15° was
always lower than that on slopes less than 15°, indicating there is a
critical slope of runoff change. This result also suggested that the
accumulative infiltration on slopes less than 15° was higher than that
on slopes more than 15°. This is not exactly the same as the results by
some study that the critical slope gradient for overland flow. For
example, A studies have found that infiltration decreased greatly with
increasing slope when the slope was less than 18°, while infiltration
was less influenced by slope beyond 18° (Cheng et al., 2008). Another
study conducted by Jin (1996) indicated that the turning point of slope
was 15°. Although the results are different, they are basically between
15° and 20° slope (Liu et al., 2017), which supports the results of the
present study.
Besides slope gradient, the difference in runoff may be also attributed
to vegetation, and interactions between slope gradient and vegetation.
Similarly, Sadeghi et al. (2013) emphasized the complex circumstances
which govern the interaction between runoff and soil loss. Some research
on the effects of vegetation cover on slope runoff erosion indicated
that when rainfall intensity reached a steady condition, runoff was
negatively correlated with vegetation cover (Joshi and Tambe, 2010;
Mohammad and Adam, 2010; Nunes et al., 2011; Zhu et al., 2010; Wang et
al., 2016), these studies only considered the effects of vegetation
cover and rainfall. However, we found that under different vegetation
covers, the changes in runoff tended to be consistent on slopes
>15°, and the runoff date curves were basically coincident
on the 20° slope, indicating the effect of vegetation cover on runoff
was more significant at medium slope gradients (5° and 10°), and slope
was more significant at steep slope gradients (15° and 20°). According
to the results of this study, the 15° slopes may be a critical threshold
for the effect of slope gradient and vegetation cover to overland flow
and soil loss.