Water Quality




Variable source area hydrology information for Educators and Students

What is Variable Source Area Hydrology?
Variable Source Area (VSA) Hydrology is the concept that runoff-generating areas in the landscape will vary in location and size over time. The VSAs are not static, they will increase and decrease in size and appear in various locations depending on time of year, rainfall, temperature, topology, and vegetation among other factors.

Why Study Variable Source Area Hydrology?
Manure application, fertilizers, pesticides, and other human - applied substances can pollute streams, especially if applied to Variable Source Areas when the ground is saturated. Since these areas are prone to runoff, substances can be easily flushed into a water system with only a small amount of precipitation. Knowing when certain parts of the landscape are prone to saturation allows land users to make better decisions regarding application of substances throughout the year; this helps keep pollutants out of streams.

Visualizing Concepts - Animations

VSA Slideshow:
View an animated clip
of saturated areas in a watershed over several years time (Note: This is a large .mpeg file)

Monthly VSA Variability Model:
Watch this animation of a modeled watershed as it wets, saturates, and then dries out over the course of a year.

Runoff Processes:

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

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