**3. Results and Discussion**

#### **3.1. Changing characteristics of soil surface roughness on the single rainfall intensity**

The changing characteristics of soil surface roughness had complicated relatively under the different rainfall intensity (Table.2). The soil surface roughness increased on the CK slope under the rainfall intensity of 0.68 mm/min. The soil surface roughness decreased on the CK slope under the rainfall intensity of 1.50 mm/min. On the PM slope, the chang‐ ing characteristics of soil surface roughness was consistent with the CK slope, however, changing characteristics of soil surface roughness decreased on the other slopes under the rainfall intensity of 0.68 mm/min. Under the rainfall intensity of 1.50 mm/min, the chang‐ ing characteristics of soil surface roughness with other slopes were contrary with the CK and showed increasing trends.

The reasons of the above results were the interaction among raindrop kinetic energy, soil surface roughness and splash amount possibly. From the angle of physics, the function of raindrop to the soil surface was one kind of acting process actually. The raindrop would hit and compact exposed soil surface when the rainfall began. At the same time, infiltrate ability of the soil reduced and soil bulk density increased gradually, and the partial soil was easy to form the crust due to soil surface fine-grain inserting in former place or migration and jam‐ ming soil pore space. Thus, soil surface roughness and the splash amounts also changed.


Note: *R* 0-soil surface roughness before rainfall, cm*R*- soil surface roughness after rainfall, cm. The same bellow.

where d is the raindrop diameter of every rainfall (mm), D is the color spot diameter (mm).

**3.1. Changing characteristics of soil surface roughness on the single rainfall intensity**

The changing characteristics of soil surface roughness had complicated relatively under the different rainfall intensity (Table.2). The soil surface roughness increased on the CK slope under the rainfall intensity of 0.68 mm/min. The soil surface roughness decreased on the CK slope under the rainfall intensity of 1.50 mm/min. On the PM slope, the chang‐ ing characteristics of soil surface roughness was consistent with the CK slope, however, changing characteristics of soil surface roughness decreased on the other slopes under the rainfall intensity of 0.68 mm/min. Under the rainfall intensity of 1.50 mm/min, the chang‐ ing characteristics of soil surface roughness with other slopes were contrary with the CK

The reasons of the above results were the interaction among raindrop kinetic energy, soil surface roughness and splash amount possibly. From the angle of physics, the function of raindrop to the soil surface was one kind of acting process actually. The raindrop would hit and compact exposed soil surface when the rainfall began. At the same time, infiltrate ability of the soil reduced and soil bulk density increased gradually, and the partial soil was easy to form the crust due to soil surface fine-grain inserting in former place or migration and jam‐ ming soil pore space. Thus, soil surface roughness and the splash amounts also changed.

**Figure 1.** Collecting board of splash erosion.

104 Research on Soil Erosion Soil Erosion

**3. Results and Discussion**

and showed increasing trends.

Relationships between rainfall energy and soil surface roughness were obtained by the method of statistics and analysis. The results followed:

Under the rainfall intensity of 0.68 mm/min: *R* 1/*R* 0=49261E-3.3451 r=0.817 n=15

Under the rainfall intensity of 1.50 mm/min: *R* 1/*R* 0=2×106 E-4.2309 r=0.836 n=15

where *R* 1 is the soil surface roughness after rainfall(cm), *R* 0 is the soil surface roughness before rainfall(cm), E is the total kinetic energy of raindrop (J/cm2 min), n is the sample number.

They had the power function relationship between the change of the soil surface roughness and kinetic energy of raindrop under the different rainfall intensities. Soil surface roughness decreased with the increasing kinetic energy of raindrop. The results had the consistent with Burwell (1969) and Steichen (1984).

#### **3.2. Changing characteristics of soil surface roughness under the combined rainfall intensity**

The combined rainfall intensity was be simulated in order to clear about the change and nature of soil surface roughness. The changing characteristics of soil surface roughness were different for the different slopes under the combined rainfall intensity (Table.3). The changing characteristics of soil surface roughness increased first, and then decreased, and increased finally with the increasing rainfall intensity on the CK slope. However, the changing characteristics of soil surface roughness increased on the PM slope, and the change of soil surface roughness increased first and then decreased on other slopes with the increasing rainfall intensity.


*<sup>R</sup>* / *<sup>R</sup>*<sup>0</sup> <sup>=</sup> <sup>−</sup>6×10−<sup>6</sup>

*P* <sup>3</sup> + 0.0012*P* <sup>2</sup> −0.0768*P* + 2.3629*r* =0.708;*n* =15 (2)

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where *R* 1 is the soil surface roughness after rainfall(m), *R* <sup>0</sup> is the soil surface roughness be‐

fore rainfall (m), *P* is the accumulated rainfall amount(mm), n is the sample number.

**Figure 2.** Relationship between change of soil surface roughness and cumulated rainfall amount.

all the slops with the increasing accumulated rainfall amount.

**Tillage practice Rainfall intensity/**

Control slope (CK)

Raking cropland (PM)

Artificial hoe (CH)

Artificial dig (TW)

Contour slope (DG)

**Table 4.** Splash erosion amounts under the different rainfall intensities.

intensities.

The changing characteristics of soil surface roughness increased first and then decreased for

Table.4 shows the splash erosion amounts of all tillage practices under the different rainfall

**Splash erosion amounts / g·cm-2**

**mm·min-1**

0.68 0.10 1.50 0.57

0.68 0.93 1.50 1.18

0.68 1.27 1.50 1.46

0.68 0.61 1.50 1.09

0.68 0.81 1.50 1.01

**3.3. Relationship between the changing of soil roughness and splash erosion amount**

**Table 3.** Change of soil surface roughness under the combined rainfall intensity.

The reasons of the above results were the interaction between raindrop kinetic energy and soil surface roughness. The micro-relief of CK slope and PM slope were relatively small in the ini‐ tial period of the rainfall. At the same time, the raindrop impact was relatively even, and they had the positive relationship between the raindrop kinetic energy and the rainfall intensity. Therefore, the changing characteristics of soil surface roughness increased and the raindrop impact gradually strengthened with the increasing rainfall intensity for the CK slope and PM slope. However, the micro-relieves of other slopes were relatively obvious in the initial period of the rainfall. At the same time, the convex fraction of raindrop impact was splashed and the concave fraction of raindrop impact was padded by other soil particle, and the part of the con‐ cave appeared the crust. So, the soil surface roughness decreased in the initial period of the rainfall. The partial soil particle of surface was dispersed or migrated, caused soil surface roughness to increase with the continuous the function of raindrop impact.

The changing characteristics of soil surface roughness were decided on the initial soil sur‐ face condition and the surface dynamic process of rainfall. The changing characteristics of soil surface roughness were analyzed with the impact of accumulating rainfall amount un‐ der the combined rainfall intensity in order to clarify the change of soil surface roughness. The results followed as Fig. 2.

Relationships between the accumulated rainfall amount and the change of soil surface roughness were obtained by the method of statistics and analysis.

$$R \,\%\,R\_0 = -6 \times 10^{-6} P^3 + 0.0012 P^2 - 0.0768 P + 2.3629 r = 0.708; n = 15\tag{2}$$

where *R* 1 is the soil surface roughness after rainfall(m), *R* <sup>0</sup> is the soil surface roughness be‐ fore rainfall (m), *P* is the accumulated rainfall amount(mm), n is the sample number.

**Tillage practice Rainfall intensity/mm·min-1** *R0/cm R/cm*

0.201

0.240

0.706

0.812

1.633

1.00 0.624 1.50 0.625

1.00 0.586 1.50 0.614

1.00 1.572 1.50 1.577

1.00 0.283 1.50 0.319

1.00 0.222 1.50 0.302

0.235

0.246

0.654

0.701

1.576

0.68

0.68

0.68

0.68

0.68

The reasons of the above results were the interaction between raindrop kinetic energy and soil surface roughness. The micro-relief of CK slope and PM slope were relatively small in the ini‐ tial period of the rainfall. At the same time, the raindrop impact was relatively even, and they had the positive relationship between the raindrop kinetic energy and the rainfall intensity. Therefore, the changing characteristics of soil surface roughness increased and the raindrop impact gradually strengthened with the increasing rainfall intensity for the CK slope and PM slope. However, the micro-relieves of other slopes were relatively obvious in the initial period of the rainfall. At the same time, the convex fraction of raindrop impact was splashed and the concave fraction of raindrop impact was padded by other soil particle, and the part of the con‐ cave appeared the crust. So, the soil surface roughness decreased in the initial period of the rainfall. The partial soil particle of surface was dispersed or migrated, caused soil surface

The changing characteristics of soil surface roughness were decided on the initial soil sur‐ face condition and the surface dynamic process of rainfall. The changing characteristics of soil surface roughness were analyzed with the impact of accumulating rainfall amount un‐ der the combined rainfall intensity in order to clarify the change of soil surface roughness.

Relationships between the accumulated rainfall amount and the change of soil surface

Control slope (CK)

106 Research on Soil Erosion Soil Erosion

Raking cropland (PM)

Artificial hoe (CH)

Artificial dig (TW)

Contour slope (DG)

The results followed as Fig. 2.

**Table 3.** Change of soil surface roughness under the combined rainfall intensity.

roughness to increase with the continuous the function of raindrop impact.

roughness were obtained by the method of statistics and analysis.

**Figure 2.** Relationship between change of soil surface roughness and cumulated rainfall amount.

The changing characteristics of soil surface roughness increased first and then decreased for all the slops with the increasing accumulated rainfall amount.

#### **3.3. Relationship between the changing of soil roughness and splash erosion amount**

Table.4 shows the splash erosion amounts of all tillage practices under the different rainfall intensities.



The change of splash erosion amounts had the difference under the different rainfall condi‐ tions on the all the slopes. The splash erosion amounts of the CK slope were lower than those of other slopes under the rainfall intensity of 0.68 mm/min and 1.50 mm/min (Table.4).

splash erosion amounts of the CK slope were lower than those of other slopes under the rainfall intensity of 0.68 mm/min, but the change of soil surface roughness was the highest (Fig.3a). However, the splash erosion amounts of the CK slope were lower than those of oth‐ er slopes under the rainfall intensity of 1.50 mm/min, and the change of soil surface rough‐

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109

The change of soil surface roughness increased first and then decreased for other slopes with the increasing splash erosion amounts under the rainfall intensity of 0.68mm/min and 1.53 mm/min. The above results were caused the interaction of raindrop kinetic energy and soil sur‐ face fluctuation condition. The raindrop impact to rainfall intensity of 1.50 mm/min was obvi‐ ously stronger than that of the rainfall intensity of 0.68 mm/min, and soil particles of former sites were sputtered. In turn, the around particle of the former sites might supplied soil parti‐ cles through the same action. The soil particles of the continuous supplement might also sup‐ ply the material base for the migration. The unceasing replacement would cause the interaction of soil surface roughness and splash erosion. So, the results were quite complicated.

Under the rainfall intensity of 0.68 mm/min, the soil surface roughness increased on the con‐ trol slope, the changing characteristics of soil surface roughness to the raking cropland slope was consistent with the control slope, however change of soil surface roughness to the other slopes decreased. The splash erosion amounts of the control slope were lower than those of other slopes, but the change of soil surface roughness was the highest. Under the rainfall in‐ tensity of 1.50 mm/min, the soil surface roughness decreased on the control slope, the change of soil surface roughness showed increasing trends on the other slopes. The splash erosion amounts of the control slope were lower than those of other slopes, and the change of soil surface roughness was the lowest. Under the combined rainfall intensity, the change of soil surface roughness of the control slope increased first, and then decreased, and in‐ creased finally with the increasing rainfall intensity. The change of soil surface roughness increased on the raking cropland slope, and the change of soil surface roughness increased

first and then decreased for other slopes with the increasing rainfall intensity.

The research was supported by the National Natural Science Foundation of China (Grant No. 40901138),National Basic Research Program of ChinaGrant No. 2007CB407201and also supported by State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Pla‐ teauInstitute of Water and Soil ConservationChinese Academy of Sciences and Ministry of

ness was the lowest (Fig.3b).

**4. Conclusions**

**Acknowledgements**

Water Resources (Grant No. 10501-283).

**Figure 3.** Relationship between soil surface roughness and splash erosion amounts under the different rainfall intensities.

The change of soil surface roughness showed the different characteristic with the splash ero‐ sion amounts under the different rainfall conditions for the all the tillage practices. The splash erosion amounts of the CK slope were lower than those of other slopes under the rainfall intensity of 0.68 mm/min, but the change of soil surface roughness was the highest (Fig.3a). However, the splash erosion amounts of the CK slope were lower than those of oth‐ er slopes under the rainfall intensity of 1.50 mm/min, and the change of soil surface rough‐ ness was the lowest (Fig.3b).

The change of soil surface roughness increased first and then decreased for other slopes with the increasing splash erosion amounts under the rainfall intensity of 0.68mm/min and 1.53 mm/min. The above results were caused the interaction of raindrop kinetic energy and soil sur‐ face fluctuation condition. The raindrop impact to rainfall intensity of 1.50 mm/min was obvi‐ ously stronger than that of the rainfall intensity of 0.68 mm/min, and soil particles of former sites were sputtered. In turn, the around particle of the former sites might supplied soil parti‐ cles through the same action. The soil particles of the continuous supplement might also sup‐ ply the material base for the migration. The unceasing replacement would cause the interaction of soil surface roughness and splash erosion. So, the results were quite complicated.
