4.2. Effects of slope gradient and vegetation cover on sediment
loss
Runoff scouring is the main motive force of sediment loss, and slope
gradient is an important factor affecting sediment loss (Zhang et al.,
2015c). In this study, the erosion sediment yield decreased and reached
a steady state with duration of scouring time under different slope
gradients and vegetation covers, and also presented an overall
increasing trend with increased slope gradient. This is because runoff
during the initial runoff producing period was large, the decrease of
infiltration resulted in the increase of runoff, so the sediment carried
by runoff increased, but it gradually reached a steady state with
increased infiltration (Wu et al., 2018). Moreover, the sediment yield
increased with increased slope gradient is mainly because the component
force of gravity increased along the down slope, thus increasing runoff
velocity (Zhang et al., 2010), and enhancing sediment transport capacity
(Li et al., 2006). Irrational production and management practices (For
example, tillage practices, sand digging and engineering construction)
destroy the riparian zone of the lower reaches of the Yellow River,
resulting in soil loss increased greatly with increased slope gradient.
The results are supported by Wang et al. (2017a) that the soil loss with
wheat stubble cover slightly increased with increased slope gradient,
and also supported by Wang et al. (2017b) that the capacity of tillage
practices to reduce runoff and sediment decreased with increased slope.
Therefore, the soil conservation measures of the riparian zone of the
lower reaches of the Yellow River should be realize coordination between
ecological protection and economic development. The idea is supported by
Ai et al. (2017), for example, by engineering measures or restoring
vegetation on severely-disturbed slopes to reduce runoff and sediment
yield.
The results of this study are similar to the findings by Li et al.
(2017) that the sediment concentration increased with increased slope
gradient and decreased with increased vegetation cover. Actually, the
effects of vegetation cover and slope gradient on erosion sediment yield
were similar to that on runoff (Joshi and Tambe, 2010; Sirjani and
Mahmoodabadi, 2014; Zhu et al., 2010). However, the erosion process
becomes more complex as result of the interaction between vegetation
cover and slope gradient. We found that the difference in accumulative
runoff sediment concentration between different vegetation covers was
obvious with increasing slope gradients. From the 10° slope, the
difference of accumulative runoff sediment concentration under different
vegetation covers was gradually larger, and it was greater than that at
the 5° slope with smaller difference of runoff sediment concentration
between different vegetation covers. This result could be related to the
relative heterogeneity of soil texture of the riparian slope, since
there was a thin layer of clay in the flat 5° slope surface, leading to
lower soil erosion under different vegetation covers. Besides, this
result also indicates that the effect of vegetation factor on runoff
sediment concentration was greater than that of slope gradient at medium
slope gradients (5° and 10°), and the effect of slope gradient was
greater than that of vegetation at steep slope gradients (15° and 20°).
On the other hand, the difference of accumulative runoff sediment
concentration between 10° and 15° slope was smaller under the same
vegetation cover, speculating there is a transition, which is caused by
the interaction between slope gradient and vegetation
cover.