*3.4. Characteristics of Soil Erosion under Different Environmental Factors* 3.4.1. Characteristics of Soil Erosion at Different Elevations

The soil erosion modulus in 1995, 2000, 2005, 2010, 2015, and 2018 was superimposed and analyzed according to different elevations, and the average soil erosion modulus at different elevations was obtained (Table 2). It can be seen from the table that the soil erosion modulus first increased and then decreased with the increase in altitude; at the height of 1494–1800 m, the slope length factor is lower, so the soil erosion modulus is lower. At the height of 1800–2100 m, there are more gullies, the slope length factor is high, andcoupled with human interference, so the soil erosion modulus is the largest. With the increase in altitude, the growth of vegetation is better, the vegetation cover and management factors are lower, and coupled with the reduction of human activities, the degree of soil intervention is low. Accordingly, the average soil erosion modulus decreased with altitude.


**Table 2.** Modulus of soil erosion in different years at different altitudes in the South and North Mountains of Lanzhou (unit: t/(km<sup>2</sup> ·a)).

3.4.2. Characteristics of Soil Erosion under Different Slopes

The soil erosion modulus in 1995, 2000, 2005, 2010, 2015, and 2018 was analyzed according to different slope grades, and the average soil erosion modulus under different slopes was obtained (Table 3). On the whole, the average soil erosion modulus was the highest on the slope of >35◦and the lowest on the slope of 0–5◦ . The 17 modulus of soil erosion increased with the increase in slope. This is mainly because the higher the slope, the greater the slope factor, and the rapid erosion of the runoff velocity caused by surface water is serious.


**Table 3.** Modulus of soil erosion in different years under different slopes in the South and North Mountains of Lanzhou (unit: t/(km<sup>2</sup> ·a)).

3.4.3. Characteristics of Soil Erosion under Different Land-Use Types

The average soil erosion modulus under different land-use types was obtained based on the regional statistical analysis of land use classification and soil erosion modulus in 1995, 2000, 2005, 2010, 2015, and 2018 (Table 4). Overall, the average soil erosion modulus of grassland and woodland was larger, which is 24.76 t/(km<sup>2</sup> ·a) and 23.43 t/(km<sup>2</sup> ·a), respectively. The average soil erosion modulus of cultivated land was 7.48 t/(km<sup>2</sup> ·a), and the average soil erosion modulus of water area was 0.27 t/(km<sup>2</sup> ·a), which is the smallest. Although grassland and woodland are covered by vegetation, grassland and woodland are relatively high above sea level, generally distributed in areas with high mountains and steep slopes, and soil erosion is more serious due to water runoff and gravity.

**Table 4.** Modulus of soil erosion in different years under different land-use types in the South and North Mountains of Lanzhou (unit: t/(km<sup>2</sup> ·a)).


3.4.4. Characteristics of Soil Erosion under Different Vegetation Coverage

Based on the regional statistical analysis of soil erosion modulus in 1995, 2000, 2005, 2010, 2015, and 2018 according to the different vegetation coverage, the average soil erosion modulus under different land-use types was obtained (Table 5). On the whole, the soil erosion modulus was the largest under low mulch. Except for bare land, the average soil erosion modulus decreased with the increase in vegetation coverage. The rise of vegetation coverage reduced rain water's splashing and running water scouring. The roots of vegetation maintained the soil and played a role in slowing down soil erosion. The average soil erosion modulus of bare land ranked third, mainly because according to the vegetation coverage of less than 10%, and the land use types are construction land and unused land., construction land was generally cement-hardened and difficult to erode; unused land generally had poor soil texture and relatively weak erosion.


**Table 5.** Modulus of soil erosion in different years under different vegetation coverage in the South and North Mountains of Lanzhou (unit: t/(km<sup>2</sup> ·a)).

3.4.5. Soil Erosion Characteristics under Different Soil Types

In ArcGIS 10.4, the South and North Mountains of Lanzhou City were classified into semi-luvisols, primarosol, pedocal, xerosol, alpine soil and anthrosol according to the definition criteria of the Chinese soil outline [45]. The soil erosion moduli in 1995, 2000, 2005, 2010, 2015 and 2018 were superimposed and analysed according to different soil types to obtain the average soil erosion moduli under different soil types. The average soil erosion modulus was highest for pedocal and lowest for alpine soil (Table 6).

**Table 6.** Modulus of soil erosion in different years at different soil type in the South and North Mountains of Lanzhou City (unit: t/(km<sup>2</sup> ·a)).


3.4.6. Comparison of Soil Erosion Inside and Outside the Environmental Greening Project

According to the statistics of soil erosion modulus in 1995, 2000, 2005, 2010, 2015 and 2018, the average soil erosion modulus were inside and outside the environmental greening project (Table 7). The average soil erosion modulus inside and outside the environmental greening project was 21.27 t/(km<sup>2</sup> a) and 23.56 t/(km<sup>2</sup> a), respectively, and the average soil erosion modulus outside the environmental greening project was larger than that inside the environmental greening project. The area of slight soil erosion inside and outside the greening area was the largest, and the area of severe soil erosion was the smallest, which was 7.72 km<sup>2</sup> and 4.11 km<sup>2</sup> , respectively, accounting for 0.39% and 0.21% of the total area. Within and outside the greening range, the area occupied by soil erosion intensity from large to small was slight, mild, moderate, strong, extremely strong and severe, respectively (Figure 10).

**Table 7.** Modulus of soil erosion in different years inside and outside the environmental greening project in the South and North Mountains of Lanzhou City (unit: t/(km<sup>2</sup> ·a)).


project in the South and North Mountains of Lanzhou City (unit: t/(km<sup>2</sup>

**1995 2000 2005 2010 2015 2018** 

outside 22.28 21.79 22.19 15.29 20.57 39.23 23.56

**Figure 10.** Average area of soil erosion intensity inside and outside the environmental greening project of South and North Mountains in Lanzhou City (unit: km<sup>2</sup> ). **Figure 10.** Average area of soil erosion intensity inside and outside the environmental greening project of South and North Mountains in Lanzhou City (unit: km<sup>2</sup> ).

**Table 6.** Modulus of soil erosion in different years at different soil type in the South and North

3.4.6. Comparison of Soil Erosion Inside and Outside the Environmental Greening Pro-

total area. Within and outside the greening range, the area occupied by soil erosion intensity from large to small was slight, mild, moderate, strong, extremely strong and severe,

**Table 7.** Modulus of soil erosion in different years inside and outside the environmental greening

According to the statistics of soil erosion modulus in 1995, 2000, 2005, 2010, 2015 and 2018, the average soil erosion modulus were inside and outside the environmental greening project (Table 7). The average soil erosion modulus inside and outside the environmental greening project was 21.27 t/(km<sup>2</sup> a) and 23.56 t/(km<sup>2</sup> a), respectively, and the average soil erosion modulus outside the environmental greening project was larger than that inside the environmental greening project. The area of slight soil erosion inside and outside the greening area was the largest, and the area of severe soil erosion was the small-

, respectively, accounting for 0.39% and 0.21% of the

·a)).

·a)).

**Soil Type 1995 2000 2005 2010 2015 2018 Average** semi-luvisols 8.47 9.33 12.77 5.79 9.89 29.00 12.54 primarosol 17.94 17.93 18.43 11.38 17.50 38.31 20.25 pedocal 21.63 17.60 21.50 10.75 15.86 41.18 21.42 xerosol 14.10 15.68 14.60 8.76 15.31 31.77 16.70 alpine soil 4.40 6.43 10.12 4.59 6.58 17.83 8.32 anthrosol 15.21 12.35 11.18 8.46 12.67 27.56 14.57

Mountains of Lanzhou City (unit: t/(km<sup>2</sup>

est, which was 7.72 km<sup>2</sup> and 4.11 km<sup>2</sup>

respectively (Figure 10).

ject

#### **4. Discussion**

**4. Discussion** From 1995 to 2018, the average soil erosion modulus in the North and South Mountains of Lanzhou showed a fluctuating downward trend. There was an abnormality in 2018, mainly due to precipitation. The natural precipitation in the North and South Mountains of Lanzhou was less and the seasonal distribution is uneven. The precipitation was mostly concentrated in July, August, and September, and its rainfall accounts for 60–70% of the annual rainfall. However, 2018 was a year of abnormal increase in precipitation. Although the average precipitation was 441.6 mm, which is less than that in other areas, the rainfall erosivity was the highest due to the low vegetation coverage and the From 1995 to 2018, the average soil erosion modulus in the North and South Mountains of Lanzhou showed a fluctuating downward trend. There was an abnormality in 2018, mainly due to precipitation. The natural precipitation in the North and South Mountains of Lanzhou was less and the seasonal distribution is uneven. The precipitation was mostly concentrated in July, August, and September, and its rainfall accounts for 60–70% of the annual rainfall. However, 2018 was a year of abnormal increase in precipitation. Although the average precipitation was 441.6 mm, which is less than that in other areas, the rainfall erosivity was the highest due to the low vegetation coverage and the collapsibility of loess. Torrential rain caused flash floods, landslides, and mudslides, resulting in the largest in soil erosion modulus in 2018.

Due to climate, natural disasters, and man-made deforestation, soil erosion and desertification in the study area are becoming more and more serious. Lanzhou's urban construction is closely related to the ecological construction of the North and South Mountains. In order to improve its ecological environment, Lanzhou started planting plantation greening projects in 1999. Therefore, most of the existing forests in Lanzhou are plantations, and a preliminary plantation ecosystem has been formed. At present, the method of increasing vegetation coverage and reducing the bare area of the ground has achieved initial results, and the soil erosion modulus within the greening project is lower than that outside the greening project.

Soil erosion moduli can be determined in a variety of ways, such as using measured runoff sediment information, simulated rainfall, field surveys, radioisotopes, and mathematical models. In this paper, the soil erosion modulus was calculated using the Revised Universal Soil Loss Equation (RUSLE). Although the limitations of the application of RUSLE in the North and South Mountions of Lanzhou City were corrected to the greatest extent possible, the RUSLE can only calculate the hydraulic erosion modulus, and parts of the North and South Mountains are in a wind erosion zone with a large area of desertification and dust storms occurring every spring [46]. As there is still a lack of effective calculation models for wind and water erosion composite soil erosion modulus, the changes in wind erosion modulus are not considered in this paper, resulting in a small calculation of soil erosion modulus in windy and sandy areas. Soil erosion is closely related to land desertification, and effective management of soil erosion can help to slow down the development of land desertification in the study area [47,48].

Countermeasures to prevent soil erosion in the North and South Mountains are the following:


(3) Increase investment in environmental greening project in the North and South Mountains and consolidate the role of the ecological security barrier in the North and South Mountains.

## **5. Conclusions**


**Author Contributions:** Conceptualization and Methodology: H.Z.; Writing–original draft, review, and editing: J.L.; Formal analysis: H.W.; Data Curation: C.X.; Visualization: Y.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** The research was funded by the Innovation and Entrepreneurship Talent Project of Lanzhou (2019-RC-105), and the National Natural Science Foundation of China (41461011).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** The study did not involve humans.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** This research received help from Chen Lei, Zhang Yuhong, An Huimin, Song Jinyue, Li Ming, and Han Wuhong from field design, sampling, and laboratory data measurement.

**Conflicts of Interest:** The funders had no role in the design of the study; in the collection, analysis, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

#### **References**

