How does water move from hillslopes into stream channels after a storm?

In hydrologic sciences, water moves from hillslopes into stream channels through two primary pathways: surface runoff and subsurface flow.12

• Surface Runoff: When rainfall intensity exceeds the soil's infiltration capacity, water collects in pools and flows over the land surface as overland flow. This process is highly efficient at generating a rapid response in stream discharge following a storm. Surface runoff is particularly characteristic of arid regions or landscapes significantly altered by human activity, such as urbanized or cultivated areas.234
• Subsurface Flow: Water that infiltrates the soil moves laterally through the subsurface toward the stream system. This movement is often slower than surface runoff. In many humid temperate regions, research using isotope tracers has revealed that a significant portion of the storm hydrograph—the record of stream discharge over time—consists of "pre-event" water. This indicates that rainfall during a storm often displaces water already stored within the catchment, highlighting the critical role of subsurface processes in storm response. Subsurface flow can be influenced by soil structure, such as clay horizons that force water to move laterally, or by macropores created by roots and animal activity.1423