**3.1 Evaluation of simulated rainfall micro-structure**

The rainfall DSD represents the major micro-structural property. Figure 4 shows the DSD measured by the piezoelectric transducer under different rainfall intensities. This result shows that the rainfall simulator generate various drop size under different rainfall intensities, which represent an advantage of the dripper-type rainfall simulators. The large drops number percentage (drops with diameter >2.5) under different rainfall intensities was calculated from results in Figure 4. The simulated rainfall large drops content (%) showed increase pattern with the rainfall intensities. On the other hand, the small drops percentage showed decreasing trend with increasing the rainfall intensities. Figure 5 shows the KE percentage at different raindrop classes (8 classes).

determining the impact of rainfall micro-structure on soil splash erosion. The splash-cups were prepared using PVC pipe-connectors with a diameter of 10 cm and height of 20 cm (Figure 3). At a height of 10 cm, a metal screen was fixed in the cup using silicon sealant (Abd Elbasit et al., 2010). A filter paper was placed on top of the screen and then the cup was filled up to the edge with silty clay loam soil collected from the Tohaku area, Tottori Prefecture, Japan. The fine sand, silt and clay percentage was 8.24, 61.78, and 29.98%,

The soil was air dried in a glasshouse and then mechanically crushed and sieved through 2 mm mesh. Before starting the experiment, the soil was again dried in an oven at 105 ºC for 24 hours. The bulk density of the soil in the cup was 1.10 ± 0.01 g cm-3. The cups were then exposed to the simulated rainfall for different durations depending on the rainfall intensity to be tested. The rainfall duration ranged from 18 minutes for 10 mm h-1 rainfall intensity to 6 minutes for 30 mm h-1. The rainfall depth was kept constant at 3 mm to avoid any surface pond formation that would have reduced the rainfall energy striking the soil surface. The splash was measured by the difference in the total oven dry weight of each splash cup

The rainfall DSD represents the major micro-structural property. Figure 4 shows the DSD measured by the piezoelectric transducer under different rainfall intensities. This result shows that the rainfall simulator generate various drop size under different rainfall intensities, which represent an advantage of the dripper-type rainfall simulators. The large drops number percentage (drops with diameter >2.5) under different rainfall intensities was calculated from results in Figure 4. The simulated rainfall large drops content (%) showed increase pattern with the rainfall intensities. On the other hand, the small drops percentage showed decreasing trend with increasing the rainfall intensities. Figure 5 shows the KE

respectively.

Fig. 3. Schematic view of splash cup.

before and after exposure to simulated rainfall.

**3.1 Evaluation of simulated rainfall micro-structure** 

percentage at different raindrop classes (8 classes).

**3. Results and discussion** 

Fig. 4. Simulated rainfall drop size distribution under various rainfall intensities.

The KE pattern was highly different from drops number percentage as the KE resulted from large drops was very high compared to small drops classes. This can be attributed to two reasons: first, the drop mass increases exponentially with diameter; second the raindrop fall velocity has a non-linear relationship with drop diameter. The small drops number and KE percentage is shown in Figure 6. The small drops number and KE percentage showed relative agreement between each other. Both the drops number and KE percentage showed a decrease with rainfall intensities. The large drops number and KE percentage showed increasing pattern with the rainfall intensities. The large drops number percentage is approximately less than 30%, however, the KE produced by this percentage of raindrops was between 70 to 90%. These results emphasize that the large drops number percentage is a determination factor for rainfall KE. The correlation coefficient between the large drops number (%) and KE (%) was 0.78 and this correlation coefficient can be improved by increasing the number of sampled intensities (Figure 7).

Fig. 5. Relationship between large drops number percentage and kinetic energy percentage under various simulated rainfall intensities.

Impact of Rainfall Microstructure on Erosivity and Splash Soil Erosion Under Simulated Rainfall 175

Fig. 8. Simulated rainfall intensity and kinetic energy relationship compared with natural

The soil splash erosion can be related directly to raindrop erosivity without any due consideration to the I-KE (Abd Elbasit et al., 2010). In other words, the rainfall erosivity can works as independent splash erosion predictor. As it was shown in the previous discussion, the rainfall micro-structure has significant effects on the rainfall erosivity and consequently on soil erosion. Figure 9 shows the relationship between splash soil erosion and large drops

Fig. 9. Splash soil erosion as a function of large drops kinetic energy percentage under

rainfall.

KE percentage.

simulated rainfall.

**3.3 Rainfall micro-structure and soil erosion** 

Fig. 6. Relationship between small drops number percentage and kinetic energy percentage under various simulated rainfall intensities.

#### **3.2 Simulated rainfall erosivity**

The rainfall erosivity has been represented in this study by the rainfall kinetic energy which was measured using a piezoelectric sensor. Figure 8 shows the relationship between the rainfall intensity and the kinetic energy (I-KE). The simulated rainfall I-KE was also compared with the natural rainfall relationships measured at different geographical locations (Figure 8). The simulated rainfall I-KE relationship showed agreement with the natural rainfall relationships under the observed rainfall intensity range (10 to 30 mm h-1). Generally, I-KE relationship showed increasing and stabilizing pattern with different thresholds. However, different patterns have been also reported by various researchers under different environments (e.g. Hudson, 1963; Carter et al., 1974). The simulated rainfall I-KE relationship, in this study, showed significant agreement with natural rainfall trends.

Fig. 7. Relationship between large drops number percentage and kinetic energy percentage under simulated rainfall.

Fig. 8. Simulated rainfall intensity and kinetic energy relationship compared with natural rainfall.

#### **3.3 Rainfall micro-structure and soil erosion**

174 Soil Erosion Studies

Fig. 6. Relationship between small drops number percentage and kinetic energy percentage

The rainfall erosivity has been represented in this study by the rainfall kinetic energy which was measured using a piezoelectric sensor. Figure 8 shows the relationship between the rainfall intensity and the kinetic energy (I-KE). The simulated rainfall I-KE was also compared with the natural rainfall relationships measured at different geographical locations (Figure 8). The simulated rainfall I-KE relationship showed agreement with the natural rainfall relationships under the observed rainfall intensity range (10 to 30 mm h-1). Generally, I-KE relationship showed increasing and stabilizing pattern with different thresholds. However, different patterns have been also reported by various researchers under different environments (e.g. Hudson, 1963; Carter et al., 1974). The simulated rainfall I-KE relationship, in this study, showed significant agreement with

Fig. 7. Relationship between large drops number percentage and kinetic energy percentage

under various simulated rainfall intensities.

**3.2 Simulated rainfall erosivity** 

natural rainfall trends.

under simulated rainfall.

The soil splash erosion can be related directly to raindrop erosivity without any due consideration to the I-KE (Abd Elbasit et al., 2010). In other words, the rainfall erosivity can works as independent splash erosion predictor. As it was shown in the previous discussion, the rainfall micro-structure has significant effects on the rainfall erosivity and consequently on soil erosion. Figure 9 shows the relationship between splash soil erosion and large drops KE percentage.

Fig. 9. Splash soil erosion as a function of large drops kinetic energy percentage under simulated rainfall.

Impact of Rainfall Microstructure on Erosivity and Splash Soil Erosion Under Simulated Rainfall 177

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