**4. Discussion**

#### **4.1 Relationship between rainfall amount, rainfall-runoff, and sediment yield**

Rainfall, vegetation, soil, and topography are the main factors involved in soil erosion [66, 67]. Based on the analysis presented here (**Table 2**), rainfall amount and rainfall intensity have the greatest effect on runoff in the semiarid loess area of Shaanxi, China. This occurs because rainfall amount and intensity are closely related to the force of erosion. If the force of rainfall increases, this can potentially have a significant effect on soil loss and runoff (**Figures 4** and **5**). This conclusion, based on the data in this study, is consistent with the findings of other scholars [9, 26, 28, 68].

While the gray relational grade values of vegetation factors were large and the relationship between runoff and sediment yield was close, vegetation had the smallest influence of all the specific indicators (**Tables 2** and **3**). This may be because vegetation coverage was high during this experiment. Vegetation coverage of runoff plots was at least 32% in 2009 (**Table 1**). After 5 years of growth, the area with the least coverage had 57% vegetation cover (**Table 1**). Therefore, the effect of vegetation on runoff may be relatively low in this study area. Others have drawn the same conclusion; that is, an increase in vegetation coverage will result in a reduction in runoff so that vegetation plays a smaller role in further reducing runoff as vegetation cover increases [69–74].

In the high runoff year, after rainfall amount and intensity, topography also had a dominating influence on runoff and sediment yield (**Figures 4** and **5** and **Table 2**). This mainly occurred because different topographic conditions led to

*Soil Erosion Influencing Factors in the Semiarid Area of Northern Shaanxi Province, China DOI: http://dx.doi.org/10.5772/intechopen.92979*

variations in soil water content in the early stage of vegetation recovery and because surface runoff differed as topography varied (**Figure 1** and **Table 1**). Low soil water content affects the infiltration capacity of soil water. If soil water content is high, soil infiltration is slow; therefore, runoff generation from excess rain leads to soil erosion [25, 26, 75–79]. The effect of rainfall intensity on runoff and sediment yield in a high runoff year was ranked from high to low, from I5, I10, I15, to I30. However, the ranking of the effect of rainfall intensity on runoff and sediment yield in most years was I30, I15, I10, and I5. Both rankings are related to the soil water content during the early stage of rainfall.

#### **4.2 Effects of land disturbance and restoration on runoff**

The weights of different factors on runoff differed significantly during PPS and PLR (**Figures 2** and **3**). **Table 3** shows that the weight significance order of the factors was topography>soil>vegetation>rainfall during PPS and the order was vegetation>rainfall>topography>soil in PLR. During PPS, the plot environments were severely degraded by trampling and digging (**Figure 2**). Slight soil disturbances do not produce serious runoff or soil erosion problems [80, 81]. In the study areas, the surface soil was destroyed, and the vegetation was heavily reduced with low vegetation coverage and canopy. The vegetation growth conditions became poor and were fragile at this time; however, stable and suitable vegetation was an effective method for reducing runoff and sediment yield [25, 82, 83].

In this study, we found that trampling and digging quantitatively decreased plant cover and vegetation, reduced soil aggregate stability, reduced soil fertility, and therefore lead to increased runoff. When the land was disturbed and the plant cover decreased, canopy interception of raindrops was low (**Figure 2** and **Table 1**). All these changes resulted in decreased mulches in the runoff plots, and thus the soil surface could not be effectively protected. This situation led to decreased rainwater infiltration and soil moisture content, and the threshold of runoff generation correspondingly decreased [39, 61, 84]. At the same time, vegetation roots were destroyed; vegetation roots can modify the structure of soil pores and can improve the soil infiltration capacity, thus reducing runoff [16, 85–88]. It has been noted that the decrease in water erosion rates with increasing root mass is exponential.

During PPS, the influence of vegetation on runoff is relatively weak, ranking the third (**Table 3**). We speculated that the protective function of vegetation on runoff was small, because during this period, all the runoff plots collected large amounts of runoff after rainfall events (**Figure 6**). Rainfall ranked fourth, for the same reason as vegetation: as long as there are rainfall-runoff events, large amounts of runoff can be produced at each runoff plot [89].

The soil surface was degraded in an irregular manner by the construction of runoff plots; therefore, the disturbances in each plot were quite different. Thus, the soil characteristics were significantly changed, especially the soil bulk density and soil steady infiltration rate. At the same time, the topography of each plot was also affected, especially the microtopography. Wilcox et al. noted that disturbances can modify surface topographical features and change the vegetation patch structure, eventually decreasing water storage within the hillslope [39]. Mohr et al. found that the impact of microtopography on surface runoff connectivity and water-repellent properties is the first-order control for hydrological and erosion processes. Therefore, during PPS, the weights of soil and topography were greater than those of vegetation and rainfall. Thus, topography and soil were major influential factors on runoff [43].

However, due to the different vegetation succession stages, the processes of runoff and soil loss are complicated and uncertain in terms of the interaction of

runoff and sediment yield were 8.59-fold and 1.70-fold than those for PLR. In addition, the fold differences between PPS and PLR for the RPa, RPb, and R vegetation types in terms of runoff were 4.81, 4.60, and 3.07; the fold differences in sediment yield were 1.35, 2.17, and 2.03. This result demonstrates that vegetation

**4.1 Relationship between rainfall amount, rainfall-runoff, and sediment yield**

Rainfall, vegetation, soil, and topography are the main factors involved in soil erosion [66, 67]. Based on the analysis presented here (**Table 2**), rainfall amount and rainfall intensity have the greatest effect on runoff in the semiarid loess area of Shaanxi, China. This occurs because rainfall amount and intensity are closely related to the force of erosion. If the force of rainfall increases, this can potentially have a significant effect on soil loss and runoff (**Figures 4** and **5**). This conclusion, based on the data in this study, is consistent with the findings of other scholars

While the gray relational grade values of vegetation factors were large and the

In the high runoff year, after rainfall amount and intensity, topography also had

a dominating influence on runoff and sediment yield (**Figures 4** and **5** and **Table 2**). This mainly occurred because different topographic conditions led to

relationship between runoff and sediment yield was close, vegetation had the smallest influence of all the specific indicators (**Tables 2** and **3**). This may be because vegetation coverage was high during this experiment. Vegetation coverage of runoff plots was at least 32% in 2009 (**Table 1**). After 5 years of growth, the area with the least coverage had 57% vegetation cover (**Table 1**). Therefore, the effect of vegetation on runoff may be relatively low in this study area. Others have drawn the same conclusion; that is, an increase in vegetation coverage will result in a reduction in runoff so that vegetation plays a smaller role in further reducing runoff as

recovery can effectively reduce the runoff and sediment yield.

*Annual runoff and sediment yield during PPS and PLR for five different vegetation types.*

**4. Discussion**

**Figure 6.**

*Soil Moisture Importance*

[9, 26, 28, 68].

**130**

vegetation cover increases [69–74].

rainfall and land use [90]. Plant growth can reduce raindrop energy and total runoff depth through canopy interception and stemflow [91, 92]. Vanacker et al. also indicated that the disturbance of vegetation cover by human activities can significantly influence erosion [46]. During PLR, the vegetation recovered, and the vegetation coverage (canopy) improved remarkably (**Table 1**). The effects of vegetation, such as the canopy interception of raindrops, the decreased velocity of raindrops, and overland flow, prevented the rainfall from directly impacting the soil surface. These effects were stronger during PLR than in PPS.

During PPS, the order of vegetation types for producing runoff was

*Soil Erosion Influencing Factors in the Semiarid Area of Northern Shaanxi Province, China*

was P > RPb > G > R > RPa. During PLR, the order of vegetation types for producing runoff was RPb > G > P > R > RPa; the order of vegetation types for producing sediment yield was P > G > RPb > RPa > R. Ai et al. found similar results

in their investigation of nine natural rainfall events [89].

flow in a loess hilly area in China [61].

*DOI: http://dx.doi.org/10.5772/intechopen.92979*

**5. Conclusion**

disturbed lands.

**133**

showed:

P > G > RPb > RPa > R; the order of vegetation types for producing sediment yield

Although the runoff and sediment yield differed for the different vegetation types, the variable coefficient for PLR was lower than that for PPS (**Figure 6**). In other words, the effect of vegetation type on soil erosion is more important during land disturbance than during land restoration or a stable vegetation period. In this study, we concluded that RPa and R were better choices for land restoration or reforestation in this area, especially for slope gradients of less than 20 degrees. A study by Chen et al. indicated that pine woodland induced the largest water loss, followed by sloping cropland, alfalfa, semi-natural grassland, and shrub land, in the Loess Plateau in China [11]. Wei et al. found that shrub species were better than grass species for retaining runoff and reducing surface water loss through overland

There are many factors that cause soil erosion; the runoff and sediment yield process is complicated. According to the research of soil erosion influencing factors in the semiarid area in Northern Shaanxi Province in China, the results

1.The order of factors affecting runoff was rainfall > soil > topography >

2.The order of factors affecting sediment yield was soil > runoff > rainfall > topography > vegetation. Soil bulk density, average rainfall intensity, and runoff had the greatest effects on sediment yield of 14 specific indicators.

3.Land disturbance and restoration significantly influence the runoff and sediment yield. The weights of influential factors (vegetation, rainfall, soil, topography) for runoff and sediment yield were also different during PPS and PLR. In this work, we determined the order of influential factor weights during PPS and PLR. This paper identified effective vegetation types for controlling runoff and reducing sediment yield. Our findings revealed that the PR and R vegetation types are better plant selections for reforestation, especially when the slope gradient is less than 20 degrees. Our research suggests that in cases of land disturbance caused by humans in semiarid regions, to quickly and effectively reduce the runoff and sediment yield, artificial measures should be taken for rehabilitation of the

The results of this study will provide an important theoretical basis for the effective reduction of soil erosion during PPS and PLR, the reconstruction of

vegetation. Rainfall amount, average rainfall intensity, and I30 had the greatest effects on runoff, based on the analysis of specific indices. Rainfall indices ranked high among the 13 specific indicators. The gray relational grade values of vegetation type, which had the smallest impact on runoff among the 13 specific indicators, was 0.5791; this large value indicates a very close relationship between vegetation and runoff in the wettest year.

As sufficient time elapsed after the disturbance, the soil and topography became basically stable (**Figure 3**). The soils of the runoff plots consolidated and became difficult to detach by runoff. Improvements in soil characteristics such as soil porosity and organic matter increased the infiltration rate and decreased the runoff. Vegetation recovery can improve soil conditions, such as soil permeability and soil water storage after rainfall, and can control runoff loss through root-network development and litter accumulation [79, 85, 93].

Once the soil and topography were stable, the vegetation restoration and rainfall features became increasingly important for runoff. Rainfall features such as rainfall duration and rainfall intensity exhibited a strong influence on runoff generation [61, 90]. Therefore, when the soil and topography were stable and the weights of these factors on runoff were low, the weights of vegetation and rainfall on runoff increased. Vegetation was a key factor in runoff, and rainfall was the second most important factor during PLR (**Table 3**).

#### **4.3 Effects of land disturbance and restoration on sediment yield**

The weights of the studied factors on sediment yield differed significantly between PPS and PLR. The weight significance order of the factors was soil > rainfall > vegetation>topography>runoff during PPS, and the order was rainfall> soil>runoff>topography>vegetation in PLR (**Table 4**). The sediment yield was different between PPS and PLR. During PPS, as a result of land disturbance, the sediment yield was greater than the land restoration. Through the observation on human activities, the removal of vegetation and disturbance of the soil surface result in the potential for soil structure degradation and sediment movement [94]. The sediment yield increases significantly for a short time after forest harvesting by clearcutting, and compared with good forest, the sediment yield is higher in sparse grass and bare areas which were without good cover [33]. As shown in **Table 4**, the effect of soil and rainfall on sediment yield ranked top two at both PPS and PLR. **Table 4** also shows that the weight of vegetation effect on sediment yield was the lowest at PLR, because during PLR, the vegetation cover in each plot was large (**Table 1**). Some scholars have found that a vegetation cover greater than 60% will significantly stabilize the soil surface and reduce soil erosion [49, 95]. During PPS, sediment transport capacity of the runoff was high; during PLR, sediment transport capacity of the runoff was low (**Figure 6**). Thus, the influence of runoff on sediment yield during PLR was greater than during PPS.

#### **4.4 Influence of vegetation type on runoff and sediment yield**

By estimating the annual runoff and sediment yield data for the five different vegetation types in each plot, we found that the runoff and sediment yield differed with respect to the different vegetation types. Similar results have also been observed by other scholars [25, 38, 61, 96–100].

*Soil Erosion Influencing Factors in the Semiarid Area of Northern Shaanxi Province, China DOI: http://dx.doi.org/10.5772/intechopen.92979*

During PPS, the order of vegetation types for producing runoff was P > G > RPb > RPa > R; the order of vegetation types for producing sediment yield was P > RPb > G > R > RPa. During PLR, the order of vegetation types for producing runoff was RPb > G > P > R > RPa; the order of vegetation types for producing sediment yield was P > G > RPb > RPa > R. Ai et al. found similar results in their investigation of nine natural rainfall events [89].

Although the runoff and sediment yield differed for the different vegetation types, the variable coefficient for PLR was lower than that for PPS (**Figure 6**). In other words, the effect of vegetation type on soil erosion is more important during land disturbance than during land restoration or a stable vegetation period. In this study, we concluded that RPa and R were better choices for land restoration or reforestation in this area, especially for slope gradients of less than 20 degrees. A study by Chen et al. indicated that pine woodland induced the largest water loss, followed by sloping cropland, alfalfa, semi-natural grassland, and shrub land, in the Loess Plateau in China [11]. Wei et al. found that shrub species were better than grass species for retaining runoff and reducing surface water loss through overland flow in a loess hilly area in China [61].

#### **5. Conclusion**

rainfall and land use [90]. Plant growth can reduce raindrop energy and total runoff depth through canopy interception and stemflow [91, 92]. Vanacker et al. also indicated that the disturbance of vegetation cover by human activities can significantly influence erosion [46]. During PLR, the vegetation recovered, and the vegetation coverage (canopy) improved remarkably (**Table 1**). The effects of vegetation, such as the canopy interception of raindrops, the

decreased velocity of raindrops, and overland flow, prevented the rainfall from directly impacting the soil surface. These effects were stronger during PLR than

development and litter accumulation [79, 85, 93].

ment yield during PLR was greater than during PPS.

observed by other scholars [25, 38, 61, 96–100].

**132**

**4.4 Influence of vegetation type on runoff and sediment yield**

By estimating the annual runoff and sediment yield data for the five different vegetation types in each plot, we found that the runoff and sediment yield differed with respect to the different vegetation types. Similar results have also been

important factor during PLR (**Table 3**).

As sufficient time elapsed after the disturbance, the soil and topography became basically stable (**Figure 3**). The soils of the runoff plots consolidated and became difficult to detach by runoff. Improvements in soil characteristics such as soil porosity and organic matter increased the infiltration rate and decreased the runoff. Vegetation recovery can improve soil conditions, such as soil permeability and soil water storage after rainfall, and can control runoff loss through root-network

Once the soil and topography were stable, the vegetation restoration and rainfall features became increasingly important for runoff. Rainfall features such as rainfall duration and rainfall intensity exhibited a strong influence on runoff generation [61, 90]. Therefore, when the soil and topography were stable and the weights of these factors on runoff were low, the weights of vegetation and rainfall on runoff increased. Vegetation was a key factor in runoff, and rainfall was the second most

**4.3 Effects of land disturbance and restoration on sediment yield**

The weights of the studied factors on sediment yield differed significantly between PPS and PLR. The weight significance order of the factors was soil > rainfall > vegetation>topography>runoff during PPS, and the order was rainfall> soil>runoff>topography>vegetation in PLR (**Table 4**). The sediment yield was different between PPS and PLR. During PPS, as a result of land disturbance, the sediment yield was greater than the land restoration. Through the observation on human activities, the removal of vegetation and disturbance of the soil surface result in the potential for soil structure degradation and sediment movement [94]. The sediment yield increases significantly for a short time after forest harvesting by clearcutting, and compared with good forest, the sediment yield is higher in sparse grass and bare areas which were without good cover [33]. As shown in **Table 4**, the effect of soil and rainfall on sediment yield ranked top two at both PPS and PLR. **Table 4** also shows that the weight of vegetation effect on sediment yield was the lowest at PLR, because during PLR, the vegetation cover in each plot was large (**Table 1**). Some scholars have found that a vegetation cover greater than 60% will significantly stabilize the soil surface and reduce soil erosion [49, 95]. During PPS, sediment transport capacity of the runoff was high; during PLR, sediment transport capacity of the runoff was low (**Figure 6**). Thus, the influence of runoff on sedi-

in PPS.

*Soil Moisture Importance*

There are many factors that cause soil erosion; the runoff and sediment yield process is complicated. According to the research of soil erosion influencing factors in the semiarid area in Northern Shaanxi Province in China, the results showed:


The results of this study will provide an important theoretical basis for the effective reduction of soil erosion during PPS and PLR, the reconstruction of

low-efficiency forests, the management of spatial vegetation, and replanting of vegetation in abandoned farmlands in the semiarid loess region.

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*DOI: http://dx.doi.org/10.5772/intechopen.92979*

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