**2.1 Study area**

hitting the soil causes water erosion [5, 6], and water erosion will occur when rainfall exceeds a certain value in a single rainfall event. Many scholars have calculated a rainfall erosion standard based on research in the loess area [7–10]. Vegetation type and coverage can reduce the soil erosion index, the effectiveness of rainfall, and the kinetic energy of raindrops and runoff and lead to different soil bulk densities [11–20]. Splash from raindrops falling on the soil surface may destroy the structure of soil by causing the displacement of soil particles (splash erosion), allowing soil movement and transportation with runoff. Therefore, particle size, bulk density, initial water content, and infiltration properties of soils have important roles in water erosion and soil loss [11, 21–26]. Topography restricts the types and configuration of vegetation and affects soil moisture, runoff production, and

runoff pathways, thus affecting water erosion and soil loss [22, 27–31].

influencing soil erosion [32–37], especially in the semiarid loess regions [11, 25, 38–41]. However, land disturbance and restoration are key factors influencing land use/cover and management [42–46]. The vegetation, root system, soil characteristics, and topography are strongly influenced by land disturbance/restoration, such as trampling and digging [46–49]. All these factors are critically important regarding runoff and sediment yield. Other human activities, such as overgrazing and defores-

tation, also increased the possibility of producing runoff and sediment yield

[48, 49].

*Soil Moisture Importance*

**120**

[31, 50–57]. Here, we use trampling as an example to illustrate the importance of land disturbance or land mismanagement. Trampling can decrease the soil macro-porosity and the associated hydraulic conductivity, thus increasing runoff production [47, 58]. Trampling can also damage plant root system, reduce vegetation coverage, and destroy soil structure, thus rendering the soil surface more susceptible to erosion

In recent years, with the implementation of the Grain for Green Program and other forestry ecological engineering, a great deal of scientific attentions has been focused on the land disturbance or/and land mismanagement and their impacts on runoff production and soil erosion. Zhao et al. studied the dynamic effects of pastures and crops on runoff production and sediment yield under simulated rainfall conditions and found that vegetation restoration can reduce sediment yield more effectively at the growing stage and can reduce runoff production more effectively at the mature stage [59]. Pan et al. investigated the influence of grass and moss on runoff production and sediment yield also under simulated rainfall conditions and found that the grass and moss can efficiently reduce sediment yield and runoff production [60]. Wei et al. studied the effects of surface conditions and rainfall intensities on runoff production using micro-runoff plots and rainfall simulation and concluded that the runoff production varies drastically with different surface conditions and also with different rainfall intensities [61]. Li et al. investigated the soil detachment capacity and its relationships with sediment yield and runoff production and found that such factors as soil aggregate median diameter, organic matter, and root density can affect soil detachment capacity and thus runoff production and sediment yield [40]. However, the effects of natural rainfall events on runoff and sediment yield were found strongly different with artificial rainfall simulations [22]. Few scholars have studied the relative weights of the four primary factors that control runoff and sediment yield. Moreover, few works in the literature focused on the weights of various factors on runoff and sediment yield during the process of land disturbance/ restoration under the conditions of natural rainfall. Above all, the specific objectives of this research were to: (1) Better understand the effects of the four factors vegetation, rainfall, soil, and topography—on rainfall-runoff and sediment yield in the semiarid loess area of Shaanxi, China, and (2) examine the characteristics of annual runoff and sediment yield under different vegetation types during a period

Land use/cover and management are considered to be the most important factors

The study area is located at the field station (**Table 1** and **Figure 1**) in Dajigou catchment, a typical loess hilly area, at northwestern Loess Plateau, Shaanxi Province, China (36°330 <sup>33</sup>″–37°24<sup>0</sup> <sup>27</sup>″N, 107°38<sup>0</sup> <sup>37</sup>″–l08°32<sup>0</sup> <sup>49</sup>″E; 1233–1809 MASL). The area belongs to the semiarid temperate climate zone. The mean annual precipitation is approximately 464.5 mm (1957–2013), of which approximately 61% falls in the summer from July to September. The monthly temperature ranges from �28.5°C (December 1967) to 38.3°C (July 2001), with an annual mean temperature of 7.9°C (1957–2013). The typical soils in Wuqi County are loess soils with relatively coarse particles [62]. The original vegetation almost disappeared. In recent years, to protect the environment in this region, the Chinese government implemented the Grain for Green Program to restore the ecological environment, namely, by restoring forest and grass vegetation. The major vegetation types are grasses, *Hippophae rhamnoides* (a spiny deciduous shrub), *Pinus tabulaeformis*, *Robinia pseudoacacia* (black locust), and other shrub and tree species. The shrub vegetation contains mixed deciduous broadleaved species (i.e., *Robinia pseudoacacia* + *Hippophae rhamnoides*) and evergreen coniferous species (i.e., *Hippophae rhamnoides* + *Pinus tabulaeformis*).
