The lateral movement of water through soil layers above the water table toward stream channels. It is an important runoff generation mechanism in vegetated hillslopes with permeable soils.
Subsurface flow, also called interflow, throughflow, or lateral subsurface stormflow, is the movement of infiltrated water laterally through the soil profile above the water table toward stream channels, springs, or seeps. It occurs when water percolating downward encounters a less permeable layer (such as a clay horizon or bedrock surface) that impedes vertical movement, causing water to move laterally along the interface. Subsurface flow is a dominant runoff generation mechanism in humid, forested, and mountainous catchments where soils are permeable enough to absorb most rainfall but where impeding layers redirect water laterally. Flow rates can be rapid when water moves through macropores such as root channels, animal burrows, and soil pipes. Subsurface flow typically generates a delayed and attenuated streamflow response compared to surface runoff, contributing to the recession limb of the storm hydrograph. It plays a critical role in maintaining stream baseflow, transporting dissolved nutrients and contaminants, and creating zones of soil saturation near streams that generate saturation-excess overland flow. Understanding subsurface flow pathways is essential for hillslope hydrology, water quality modeling, and landslide hazard assessment.