Conditions for lateral downslope unsaturated flow and effects of slope angle on soil moisture movement

SummaryThe necessary conditions for lateral downslope flow in the unsaturated zone in hillslopes have not been clearly identified, and the effects of different slope angles on soil moisture movement are rarely studied. These questions were investigated through drying processes in a homogeneous and isotropic sloping soil tank in this study. Three experiments with different slope angles were conducted. The results showed that the flow direction in the unsaturated zone gradually rotates counterclockwise from the vertical direction to the lateral downslope direction parallel to the slope surface during the drying phase. The lateral downslope flow parallel to the slope surface first appears at the surface (0–5 cm) and the bottom of the sandy loam in the tank. The lateral downslope flow parallel to the slope surface at the surface is due to the state of drying rather than the no-flow upper boundary condition. The lateral downslope flow parallel to the slope surface at the bottom of the sandy loam is also caused by the state of drying. More importantly, the results of this study showed that the rate at which the lateral downslope flow turns to be parallel to the slope surface is affected by the drainage rate. In contrast to the lateral downslope flow component parallel to the slope surface, the flow component normal to the slope surface is less sensitive to changes in the slope angle. The influence of the slope angle on the flow component normal to the slope surface is greatest in the middle layer.

► Lateral downslope flow at slope surface and bottom is due to the state of drying.
► Speed of lateral flow to become parallel to slope surface is due to drainage rate.
► Lateral flow component parallel to slope surface is more sensitive to slope angle.
► Influence of the slope angle is the most significant in the middle soil layer.