Is overland flow the main source of streamflow in storms?

Not always. In many humid, vegetated catchments most storm runoff comes from saturated near-channel areas and from rapid subsurface flow rather than widespread infiltration-excess overland flow, which is more typical of arid or disturbed land.

What is a variable source area?

It is the part of a catchment, usually near streams and in wet hollows, that becomes saturated and generates runoff during a storm; this area expands as rain continues and contracts as the catchment dries, so the runoff-producing area is dynamic.

Runoff Generation Mechanisms

Runoff generation mechanisms are the hillslope processes, infiltration-excess and saturation-excess overland flow and subsurface stormflow, by which rainfall and snowmelt become streamflow.

Definition

Runoff generation mechanisms are the physical processes by which water reaches a stream channel during and after a storm, principally infiltration-excess overland flow, saturation-excess overland flow, and subsurface stormflow.

Scope

This topic covers the distinct pathways that turn precipitation into runoff on hillslopes, the conditions under which each dominates, and the variable-source-area concept that describes where runoff originates within a catchment. It explains the physical origin of the storm response that the hydrograph and unit-hydrograph topics describe quantitatively.

Core questions

What are the main mechanisms that generate storm runoff?
When does infiltration-excess overland flow dominate versus saturation-excess?
How does subsurface stormflow contribute to the storm response?
Where within a catchment is runoff generated?

Key concepts

Infiltration-excess overland flow
Saturation-excess overland flow
Subsurface stormflow
Variable source area
Partial area contribution
Return flow

Key theories

Infiltration-excess (Hortonian) overland flow: When rainfall intensity exceeds infiltration capacity, the excess flows over the surface; Horton's mechanism dominates in arid, disturbed, or low-permeability settings where infiltration capacities are readily exceeded.
Saturation-excess flow and variable source areas: In humid, vegetated catchments runoff is generated where the soil saturates from below, with rain on these expanding saturated, or variable source, areas running off; this reframed runoff as coming from dynamic partial areas.

Mechanisms

Infiltration-excess overland flow occurs when rain falls faster than the soil can absorb it, so the surplus runs off the surface. Saturation-excess flow occurs where the soil profile fills with water from below, typically near channels and in topographic hollows, so that further rain cannot infiltrate and runs off; these saturated areas expand and contract, giving variable source areas. Subsurface stormflow is the rapid lateral movement of water through permeable soils to the stream.

Clinical relevance

Identifying the dominant runoff mechanism is essential for predicting flood response, evaluating how land use, soil compaction, and forest change alter runoff, targeting source areas for water-quality and erosion control, and structuring rainfall-runoff models appropriately.

History

Horton's infiltration theory dominated until field experiments in the 1960s and 1970s by Hewlett, Dunne, and others demonstrated saturation-excess runoff and subsurface stormflow in humid, forested catchments, replacing a single universal mechanism with a recognition that the dominant process varies with environment.

Debates

Dominant runoff mechanism: The long-running debate over whether storm runoff is primarily Hortonian infiltration-excess or saturation-excess and subsurface stormflow was resolved into a context-dependent view in which climate, soils, vegetation, and land use determine which mechanism dominates.

Key figures

Robert E. Horton
Thomas Dunne
John D. Hewlett

Seminal works

horton1933
dunne1970
hewlett1967

Frequently asked questions

Is overland flow the main source of streamflow in storms?: Not always. In many humid, vegetated catchments most storm runoff comes from saturated near-channel areas and from rapid subsurface flow rather than widespread infiltration-excess overland flow, which is more typical of arid or disturbed land.
What is a variable source area?: It is the part of a catchment, usually near streams and in wet hollows, that becomes saturated and generates runoff during a storm; this area expands as rain continues and contracts as the catchment dries, so the runoff-producing area is dynamic.