Darcy's law is a constitutive equation that explains how fluids are able to flow through a porous medium, such as a rock or sand. This law can be used to describe how water flows through aquifers and reservoirs.
The equation demonstrates that the amount of flow a fluid has between two points shares a directly proportional relationship to the distance between the two points, the pressure difference between the two points and the permeability of the flow paths in the rock. Darcy's law is only valid when the fluid's flow is slow and viscous. This law is useful in earth sciences because most groundwater flow is slow and viscous.