Darcy's Law

Darcy's Law says that groundwater moves faster when the porous material is more permeable, when the flow cross-section is larger, and when the difference in hydraulic head is steeper over a given distance. The negative sign shows that water flows from higher head to lower head, so the flow direction is down the hydraulic gradient. It is an empirical relationship that works well for laminar flow through saturated porous media, where the tiny connected pore spaces behave like many small flow channels.

Use this when estimating groundwater discharge through soil, sand, gravel, or fractured rock, especially in hydrogeology and environmental engineering. It is also useful for comparing aquifer properties, predicting seepage through embankments, or analyzing contamination transport in saturated subsurface flow.

• Using the formula for unsaturated flow without checking that the pore space is fully saturated
• Forgetting that Δh/L is a gradient, so units must cancel to give a pure number
• Ignoring the negative sign and losing the flow direction from high head to low head