1. INTRODUCTION
Soil erosion is wide-ranging worldwide and has irreversible effects on
all-natural and artificial ecosystems (Fu et al., 2011). It is not only
the primary cause of soil deterioration (Marques et al., 2008), land
productivity decline (Lantican et al., 2003), and degradation of rivers,
lakes and estuaries but also often carries sediments and pollutants.
Soil erosion has accelerated approximately 85% of global land
degradation and resulted in a 17% reduction in food production (Tang et
al., 2015), which has become one of the most serious societal and
environmental problems in the world.
The middle and upper reaches of the Yellow River basin are located on
the Loess Plateau of China. Owing to its unique soil characteristics,
climate conditions and extremely fragile ecosystem (Wang et al., 2017;
Xin et al., 2012), the Loess Plateau is a global hot spots of soil
erosion. Approximately 40% of the whole Loess Plateau suffers from
extremely high erosion, and the annual soil erosion modulus is more than
5000 t/km2 (Fu et al., 2011), which increases the risk
of soil degradation and restricts the sustainable development of the
ecosystem (Zhao et al., 2016). The high-risk areas are mainly
concentrated in hilly and gully loess areas (Chen et al., 2007), which
are the main source of Yellow River sediment. Since the 1950s, to
control severe soil erosion, the government has issued many policies and
implemented a series of soil conservation measures for the Loess
Plateau. The control of soil and
water loss on the Loess Plateau can be divided into two parts, namely,
slope vegetation restoration (i.e., reforestation) and gully control
(i.e., check dams).
As a main measure for reducing
erosion, vegetation restoration has been widely used on the Loess
Plateau, especially since the implementation of
the
‘Grain
for Green’ project. The increase in vegetation coverage can effectively
control soil erosion through root consolidation of soil, increased
infiltration, and reduced surface runoff and water flow velocity
(Bruijnzeel, 2004), improving the resistance of the soil erosion and
reducing water erosion capacity and sediment transport capacity (Rey et
al., 2005; Vanacker et al., 2007). At the same time, the variation in
vegetation cover will directly lead to changes in land use.
In
fact, land use changes will be affected by human activities, such as the
implementation of policies and urban development and its impact on soil
erosion is bidirectional. Changes in land use may not only lead to
sediment reduction (Choukri et al., 2020) but also increase soil erosion
and further lead to land degradation (Aneseyee et al., 2020). For
example, Quiñonero-Rubio et al. (2016) used the WaTEM/SEDEM model to
simulate the Upper Taibilla catchment of Spain and indicated that
afforestation reduced sediment yield by 13.9% (1956-2000).
Aneseyee et al. (2020) demonstrated
that land use changes increased soil loss by 26.25% and sediment export
by 3.45% (1988-2018), mainly due to the expansion of urbanization and
reclamation.
The construction of check dams has become an effective soil and water
conservation measure for sediment control. A large number of check dams
have been constructed in the gully channel to intercept the sediment,
block floods, and reduce downstream scouring (Ran et al., 2008).
When the check dams are fully filled,
the main component of the land behind check dams is surface soil that is
rich in nutrients, and its fertility is much higher than that of sloped
farmland. According to reports, approximately 3200 km2of dam croplands were formed in 2002 (Jin et al., 2012). Moreover, check
dams effectively prevent sediment from entering the Yellow River, thus
reducing the sediment concentration of the Yellow River (Ran et al.,
2008). Although not widely used worldwide, check dams have been applied
to control soil erosion and have been reported in France, the United
States, Spain, China and elsewhere (Abedini et al., 2012; Bellin et al.,
2011; Borja et al., 2018; Castillo et al., 2007; Fang, 2017). Polyakov
et al. (2014) carried out field sampling in the Santa Rita Experimental
Range in the United States and found that the check dams retained 50%
of sediment yield. By measuring the sediment deposition within the gully
channels of the Loreto catchment in the Andean Mountains, Borja et al.
(2018) indicated that check dams reduced the sediment exported by more
than 70%. Boix-Fayos et al. (2008) applied the WATEM-SEDEM model to the
Rogativa catchment of Spain and showed that check dams reduced the
sediment load by approximately 77% without land use changes. Xu et al.
(2013) explored the interception benefit of check dams in the Yanhe
River watershed and revealed that the proportional reduction in sediment
reached from 34.6-48.0% in the rainy season (1984-1987) and increased
to between 79.4 and 85.5% from 2006-2008.
In the past few decades, the Loess
Plateau has also undergone varying degrees of climate change, in which
precipitation variation plays a leading role in soil erosion.
Sun
et al. (2015) analyzed the precipitation changes of the Loess Plateau
from 1961 to 2011, and found that the amount of precipitation in most
areas showed a downward trend. In
addition, over the past six decades, the sediment concentration of the
Yellow River has decreased substantially (Wu et al., 2020),
and
the main tributaries in the middle reaches of the Yellow River have
reported that sediment load has decreased rapidly (Gao et al., 2017;
Rustomji et al., 2008; Yue et al., 2014).
Hence, it is particularly
important to understand the driving factors and mechanisms behind the
changes in soil erosion and sediment load, which is also a prerequisite
for sustainable watershed management (Montgomery,
2007).
To quantitatively identify the impact of the three factors
(precipitation variation, land use
changes and check dams), the Kuye River watershed, with an area of 8651
km2 on the Loess Plateau, was selected as the study
area by combining field investigations with model simulation. The main
objectives of this study were to (1) explore the characteristics of soil
erosion and sediment yield and to (2) quantify the contribution of
precipitation variation, check dam construction and man-made land use
changes to sediment reduction in the Kuye River watershed, which is a
typical watershed in the coarse sandy hilly catchment region of the
Yellow River basin.