Effects of tillage measures on runoff
We verified that crops could act as a type of vegetation cover (Table 1
and 2) and play an important role to mitigate runoff and soil loss on
sloping farmlands in agreement with previous studies (Cerdà et al.,
2017; Keesstra et al., 2016; Prosdocimi et al., 2016a, b; Wang et al.,
2018). Different tillage systems had different impacts on soil erosion
associated with processes occurring in slope farmlands (Liu et al.,
2011; Xu et al., 2018). The Vr treatment has already been verified to
increase soil erosion because of microtopography changes (Liu et al.,
2011; Zhang et al., 2009). Conservation tillage could significantly
postpone runoff yield and slow
runoff velocity compared to conventional tillage (Table 3). Our results
indicated that horizontal ridges, mulching, or seedling corn canopy were
effective in controlling runoff generation (Table 1, 2 and 3),
especially at 50 mm/h, at the maize seedling stage. This might be
because the studied conservation measures could enhance the infiltration
capacity of water (Table 5) or increase soil surface roughness
(Rodríguez-Caballero et al., 2012; Vermang et al., 2015; Wang et al.,
2018), and crop leaves could intercept rainfall and change raindrop
diameter and energy (Table 1 and 2) (Ma et al., 2013; Zhang et al.,
2015). As there are only limited chances for extreme precipitation in
the region (Zhang et al., 2010), adopting Hr and Cm would be able to
restrict runoff generation. In addition, these two tillage measures were
also efficacious in reducing the
runoff-flow
velocity, which is a key factor
affecting runoff energy and erosiveness (Vermang et al., 2015); both Hr
and Cm performed better at 50 mm h-1 than at 100 mm
h-1. Our results were in accordance with previous
studies on other soil types (Prosdocimi et al., 2016a, b; Xu et al.,
2017). The runoff generation was postponed and the surface-flow velocity
decreased mainly because both Hr and Cm treatments changed the
microtopography of the soil with increasing surface roughness (Vermang
et al., 2015; Wang et al., 2018) and the infiltration of conservation
tillage was higher than that of conventional measures (Table 3). The
outcome provided more water storage microstructure for the surficial
soil, causing the rainwater to infiltrate rather than flowing downhill
(Liu et al., 2015; USDA-ARS, 2008,
2013). This outcome also increased
the friction between rainwater and land, thereby reducing runoff
velocity. Comparing the effects of
Hr and Cm, Hr set a higher threshold for runoff yield, as it could lead
to more water storage between ridges. However, once the runoff had
occurred, Cm performed better, since the presence of cornstalk could
reduce the flow velocity to a very low level (Table 1). Thus, Hr+Cm is
the optimal treatment from the perspective of postponing runoff-yield
and restricting the destruction of runoff, once
generated.
The runoff loss rate significantly increased following a low start
during the runoff generation period and then remained stable at a
certain level, based on the rainfall intensity. The results correspond
to a study on purple soil
(Xu et al., 2008). Hr and Cm could
effectively constrain the runoff loss rates and decrease the runoff
amount, especially at 50 mm h-1. The Hr+Cm treatment,
which combined horizontal ridging and mulching, always had an effective
influence on runoff under all rainfall types, especially for preventing
runoff throughout the experiment under the rainfall intensity of 50 mm
h-1. As runoff is
the main vector affecting both soil loss and agricultural nonpoint
source pollution (Hudson, 2015; Zhang et al., 2007), Hr+Cm should be
recommended as an effective tillage practice in the region.
However, this recommendation would engender extremely higher outliers
for runoff rate as a real-time response to ridge rupture when the plots
were treated with Hr, especially
under heavy rainfall conditions (Li et al., 2016; Lu et al., 2016). In
this case, the water held by the two adjacent ridges drained immediately
after ridge rupture and outrushed into the next inter-ridge area,
causing either successive ridge ruptures or runoff overflow, both of
which could prompt a sudden upsurge in runoff rate, in accordance with
the results of Xu et al. (2018). Consequently, the total runoff loss
amount also ascended. The rising magnitude caused by ridge rupture
depended on the rupture time and location of the initially ruptured
ridge. For instance, in the Hr-treated plot, ridge rupture occurred
relatively earlier and closer to the top of the plot under the rainfall
intensity of 100 mm h-1 than under 50 mm
h-1 (Figure 6 and 7), thereby resulting in greater
runoff loss. Thus, enhancing the quality of ridges to improve their
water pressure tolerance capability is
vital when applying horizontal
ridges (Liu et al., 2014a). It was found that mulching could directly
lead to water absorption and protection of a ridge from saturation and
erosion by raindrop and runoff (Cerdà et al., 2016; Jordán et al.,
2010), thereby reducing the risk of ridge rupture. The results confirmed
that the Hr-treated plots suffered three times as many ridge ruptures,
while the Hr+Cm plots suffered only one ridge rupture. Moreover, no
successive ridge ruptures were observed in the Hr+Cm plots, because
mulching and soil blocks would likely be obstructed by the next ridge
with the presence of cornstalk, rather than triggering successive ridge
ruptures, even if one of the ridges happened to rupture. Moreover,
ridge-furrow planting under mulching conditions played an effective role
in reducing surface runoff with an increase in soil-water infiltration
(Gholami et al., 2013; Kader et al., 2017).
Vr could increase the runoff loss rate and amount under light rainfall
conditions, as shown by Shen et al. (2005) and Zhang et al. (2009) on
black soil, and by Xu et al. (2008)
on purple farmlands compared to the runoff between contours and
downslope ridges. Therefore, vertical ridges should be avoided on slope
croplands in the region.