4.2.1. Sandy Soil

The variations of volumetric water content of sandy soil for different sections under different anisotropy direction α values are shown in Figure 5.

For the top of slope, the volumetric water content on the surface decreased with the increase of α. This is because *kx* was greater than *ky*. When α = 0◦, the vertical permeability reached the minimum, so the rainfall was hard to infiltrate, and the rain water accumulated in the shallow part of the slope. With the increase of α, however, the vertical permeability increased, and the rain water was easier to infiltrate into the deep part, thus leading to the decrease of the surface volumetric water content. What should be noticed is that when the anisotropy ratio was small (i.e., *kr* = 10), the impact of rainfall on the volumetric water content was mainly reflected on the slope surface; but when the anisotropy ratio was larger (i.e., *kr* = 100), not only the slope surface but also the deep area were violently influenced, especially for α = 0◦ and α = 15◦.

For the middle of slope, the volumetric water content was affected by the combined effect of rainfall infiltration and the rainfall excretion from the slope top. The increase of α also decreased the

volumetric water content on the surface as was illustrated in the previous paragraph. The combined effect, however, made the maximum surface volumetric water content happen at α = 15◦.

For the toe of slope, the variation of volumetric water content was similar to the slope top and middle In the condition of α = 0◦, however, the whole section did not reach saturation with *kr* = 10, but reached saturation with *kr* = 100.

Figure 6 shows the variation of volumetric water content under di fferent *kr* values. For the top of slope, the volumetric water content under di fferent *kr* values was greatly a ffected by the values of α. When α was relatively small (i.e., α = 0◦), the surface volumetric water content increased with the increase of *kr*. This is because the increase of *kr* decreased the horizontal permeability, thus leading to a higher surface volumetric water content. Meanwhile, rain water was easier to infiltrate through the horizontal direction when *kr* decreased, which rose the underground water level, thus leading to the increase of the deep volumetric water content. Yeh H.F. et al. (2018) [44] conducted relevant.simulations and the results were similar to current research results. However, the increase of α values made the di fference of volumetric water content between di fferent *kr* values smaller. This is because the vertical permeability decreased with the increase of α, and the rain water drained away rather than infiltrated directly into the slope soil.

For the middle of slope, the variation range of volumetric water content was larger than that of the slope top, and under the combined e ffect, the value of volumetric water content was also larger than that of the slope top.

For the toe of the slope, due to its lower terrain, the height of the initial water table to the surface of the slope toe was smaller, and it was a ffected not only by rainfall but also by the rain water from the slope middle. So it had a smaller unsaturated area and a larger volumetric water content. What should be noticed is that the whole section reached saturation when α = 0◦ and *kr* = 50 and 100, and did not reach saturation in other conditions.
