# How to Calculate Groundwater Velocity ••• Thomas Northcut/Photodisc/Getty Images

An accurate estimate of groundwater velocity can be calculated using Darcy’s Law. Darcy’s law is an equation that describes groundwater movement in aquifers based on three variables: horizontal hydraulic conductivity, horizontal hydraulic gradient and effective porosity. The equation for calculating ground water velocity is: V= KI/n.

In this formula V stands for "groundwater velocity," K equals the "horizontal hydraulic conductivity," I is the "horizontal hydraulic gradient," and n is the "effective porosity."

Determine the horizontal hydraulic conductivity, which is the ease at which groundwater can move through pore space and fractures in the soul. The value (K) can be found in the United States Department of Agriculture, Natural Resource Conservation Service's Soil Survey Manual. Select the appropriate value based on the class of soil.

Determine the applicable horizontal hydraulic gradient. This value can be established by measuring water levels. Horizontal hydraulic gradient is simply the slope of the water table. It is the change in hydraulic head over the change in distance between the two monitoring wells or dh/dl.

In mathematical terms, horizontal gradient is rise over run; dh/dl equals the difference in head divided by horizontal distance between wells.

Determine the effective porosity. Attached is an effective porosity table that you can pull the correct effective porosity that is right for your calculation, based on the type of soil and soil characteristics.

Verify that the units correspond; multiply the horizontal hydraulic conductivity with the horizontal hydraulic gradient. Then divide the product by the effective porosity. The result is the groundwater velocity.

#### Tips

• Using a spreadsheet to calculate groundwater velocity can help save time and limit the potential for error.

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