throughflow

Throughflow is the lateral movement of water within the soil or regolith, moving downslope at shallow depths and often toward a stream network. It is a component of the hydrological cycle that occurs after rainfall infiltrates the soil and is distinct from vertical percolation to groundwater and from surface runoff.

Water reaches throughflow channels via macropores created by earthworms and plant roots, cracks, and layers with higher hydraulic conductivity than surrounding material. Preferential flow can cause rapid transport along these pathways, while matrix flow contributes more slowly through the soil. Throughflow can occur in perched layers above less permeable horizons, forming subsurface conduits that guide water to streams.

Throughflow is influenced by slope gradient, soil permeability and structure, antecedent moisture, rainfall intensity, vegetation cover, and land use. It tends to be greatest on structured soils with macropores and on moderately steep slopes, and it can diminish where layers are homogeneous or surface protection reduces infiltration.

It affects the timing and magnitude of runoff, contributes to baseflow in streams during dry periods, and serves as a conduit for nutrients and contaminants from the landscape into waterways. In hillslope hydrology, throughflow can produce rapid responses to rainfall events and influence erosion and sediment transport.

Field estimation uses tracer tests, seepage or lysimeter measurements, and soil moisture monitoring, often combined with hydrological modeling to delineate interflow or throughflow components. In hydrology, terms such as interflow or lateral flow are closely related and sometimes used interchangeably depending on the scale and terminology.