Water Quality




Information for extension educators,
management professionals, and public

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.

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

VSA Hydrology is an important concept for land use managers to consider when trying to minimize the transport of agricultural nutrients and pollutants to receiving water bodies. Amendments to the Clean Water Act (1972), especially those in 1987, increasingly emphasized nonpoint source (NPS) pollution as a critical cause of water quality degradation. However, agricultural practices and management strategies for reducing NPS pollution are generally lagging the scientific state-of-the-art by several decades.


Below is a collection of information and tools that land managers may find useful for protecting surface water from excess agricultural pollution:

Interactive VSA Utility for Land Use Management

Researchers in the Department of Crop and Soil Sciences at Cornell are developing an interactive tool for land use managers to identify hydrologically sensitive areas in New York State. To access a demo of this tool, visit:

Tompkins County Saturated Areas Locator
A Demonstration Internet Map Service
Simple monthly saturation probability is interactively mapped in the Tompkins County area. Users may zoom into a specific area and select the month and likelihood of saturation (as a percent) to display. Areas likely to be saturated are highlighted in red. This service is provided as a demonstration only and is currently very slow to run, as it is not running on a dedicated IMS server yet.

The Soil Moisture Distribution and Routing Model

The Soil Moisture Distribution and Routing Model, SMDR or SMR, is a physically-based, fully-distributed, hydrological model which has been developed in the Soil and Water Lab for the last 10 years. SMR is a Geographic Information System (GIS)-based mass balance model for simulating soil moisture routing and distribution based on spatially distributed soil hydrodynamic properties. The SMDR website provides more information about this model and offers free access to updated executables.

Phosphorous Runoff: P-Index

The P-Index is a measure of phosphorus concentration and movement within a landscape and is primarily used in agricultural fields in nutrient management planning. Visit the Cornell Crop and Soil Sciences Nutrient Management Spear Program website for Agricultural P-Index information:

Selected References

Hydrologically Sensitive Areas: Variable Source Area Hydrology
Implications for Water Quality Risk Assessment
by M.Todd Walter, Michael F. Walter, Erin S. Brooks, Tammo S. Steenhuis, Jan Boll, Kirk Weiler Download: PDF Format (385 kb)

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