VSA Processes

Variable Source Area Hydrology is the concept that the majority of runoff exiting a watershed is driven by a relatively small portion of the watershed. The abundance of these runoff generating sources may increase and decrease depending on temporal and spatial variability.

Agricultural field conditions in August (Dry)	Agricultural field conditions in March (Wet)

The contribution to runoff by these variable source areas is governed by two major processes:

• Saturation Excess Overland Flow
• Infiltration Excess Overland Flow (also known as Hortonian Flow)

Animated VSA Hydrology Processes
(Examples of Saturation Excess Overland Flow):

• Increased interflow levels due to precipitation inputs
• Elevated water table due to increased water inputs
• Exceeded water holding capacity in shallow soil

Watch a model of VSAs throughout the year:

• Monthly VSA Model Animation