Why do steep, humid, forested hillslopes sometimes produce subsurface storm flow?

In the context of hydrologic sciences, subsurface storm flow occurs on steep, humid, forested hillslopes primarily due to the interaction between soil structure and topography.

When precipitation infiltrates the soil, it moves downward until it encounters a less permeable layer, such as a well-developed clay horizon (Bt). Because this layer acts as an obstacle to vertical percolation, the water is forced to flow laterally along the horizon toward the stream system.1

This process is further influenced by several key factors:

• Macropores: The presence of macropores—created by plant root growth and decay, animal burrowing, or soil fracturing—significantly facilitates the movement of subsurface water.1
• Saturation: In humid regions, this lateral movement often leads to the saturation of the upper soil profile. When the soil's water-storage capacity is exceeded, the excess water contributes to streamflow.12
• Topography: The configuration of the slope, such as concave-upward areas, often dictates where water accumulates, influencing the drainage patterns and the development of specific soil horizons that promote lateral runoff.3

This subsurface process is characteristic of humid environments, where it serves as a primary mechanism for moving water through the landscape.1