Groundwater flow is from high hydraulic head (high water level) to low hydraulic head (low water level). Calculations of groundwater flow rate Q, can be made by Darcy's Law. Henri Darcy, a French engineer, was in charge of drinking water treatment in Dijon in the 1800s. Treatment involved passing water through sand filters. Darcy conducted experiments into the parameters controlling the water flow rate through the filters.

Darcy found that the cross-sectional area (A), difference in water level from the entrance to the exit of the filter pack (H1-H2) and length of the filter pack (L) had the following relationship to water flow rate, Q, for a given sand.

Q proportional to A

Q proportional to H1-H2

Q proportional to 1/L

As he increased the cross-sectional area, A, the flow rate increased.

As he increased the difference in water levels, the flow rate increased.

As he increased the filter pack length, the flow rate decreased.

For that particular sand in the filter, he could use a proportionality constant, K, which allowed him to use the following equation to calculate flow rates, Q.

Q = K A [H1-H2]/L

"Darcy's Law"

The term, [H1-H2]/L, is grouped together and referred to as the hydraulic gradient. This is the slope of the water piezometric surface.

K was a property of the type of geologic material in the sand filter. Setting up an experiment similar to Darcy's, you can measure Q, A, H1-H2 and L. Using Darcy's Law, you can then calculate K from your experiment. Such experiments are called permeameter measurements.