## 3 Results and Discussion
### Soil Water Change in Slopes under Natural Evaporation Conditions  
#### 3.3.1 The Law of Total Variation in Slope Soil Water under Natural Evaporation Conditions  
The diurnal variation in soil water content at different depths during the natural evaporation stage is depicted in Figure 12. For ease of illustration, the figure includes the soil water content before rainfall. It can be observed that the initial water content of slopes with the same slope protection mode is relatively similar at different slope ratios and depths before rainfall. When the initial water content is high, its impact on the soil water field distribution of the slope is somewhat limited [18]. Hence, the impact of the initial water content on the distribution of the soil water content in the profile can be disregarded. The test results shown in Figure 12 indicate that the soil water content at each depth reaches its maximum within the first day after rainfall. With the exception of a few vegetated slopes, the soil water content 14 days after rainfall is higher than before the rainfall but gradually decreases to the level before the rainfall. As natural evaporation continues after the rainfall, the potential energy of soil water in the upper layer decreases. Consequently, soil water starts to move slowly upwards in the form of capillary water, primarily influenced by the soil matrix potential [19]. As time passes after rainfall, the rate of water migration in the soil gradually slows down and the soil water content tends to stabilize. Figure 11: The average increase in soil water content at different depths of the vegetated slope and how it varies with time: (**a**) slope ratio of 1:1.5, (**b**) slope ratio of 1:1.75, (**c**) slope ratio of 1:2. Figure 12: The soil water content of bare and vegetated slopes with different slope ratios in the natural evaporation stage and how it varies with depth within 14 days: (**a**) bare slope, 1:1.5; (**b**) vegetated slope, 1:1.5; (**c**) bare slope, 1:1.75; (**d**) vegetated slope, 1:1.75; (**e**) bare slope, 1:2; (**f**) vegetated slope, 1:2. In the process of natural evaporation, Bermuda grass has a marked effect on soil water content, with its range of influence reaching over 100 cm. This influence is more pronounced within the top 40 cm of soil depth but becomes less significant at depths greater than 60 cm. When the distance from the slope surface is 100 cm, the influence of rainfall on soil water content is greater, while evaporation has a limited impact. The soil water content of the bare slope decreases in the following order from the slope surface: 100 cm, 40 cm, 20 cm, and 60 cm. Conversely, the soil water content of the vegetated slope decreases in the following order from the slope surface: 100 cm, 20 cm, 40 cm, and 60 cm. This difference is due to the presence of a thicker matrix epiphytic layer in the topsoil (10-30 cm) of herbaceous plants, which has a higher water-holding capacity [20]. Consequently, the soil water content at a depth of 20 cm is higher in the vegetated slope compared to a depth of 40 cm. This is in contrast to the soil water distribution in the bare slope. Bermuda grass on vegetated slopes has the ability to enhance slope water redistribution. It promotes water infiltration, leading to faster natural evaporation efficiency [21; 22]. After rainfall, the vegetated slope with the smallest slope ratio experienced the greatest increase in soil water content at each depth. The soil water content at depths of 20 cm, 40 cm, 60 cm, and 100 cm increased from 28%, 26.8%, 25.5%, and 37.1% to 38.9%, 34.6%, 31.5%, and 39.9%, respectively. During the natural evaporation stage, the soil water content on the vegetated slope with the steepest incline decreased from 31.8%, 28.8%, 26.5%, and 31.3% to 25.1%, 25.1%, 24.6%, and 31.1% at depths of 20 cm, 40 cm, 60 cm, and 100 cm. This slope ratio experiences the largest reduction in soil water content among all slope ratios.