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.