Darcy's Law directly calculates the discharge velocity. To get the actual average groundwater velocity (seepage velocity), what specific aquifer property is essential to account for?

To get the actual average groundwater velocity, also known as seepage velocity, from the discharge velocity calculated by Darcy's Law, the essential aquifer property to account for is porosity. Darcy's Law calculates discharge velocity, which represents the volume of water flowing per unit time through a unit total cross-sectional area of the aquifer, including both solid grains and void spaces. This is an apparent or superficial velocity, averaged over the entire aquifer material. However, groundwater only flows through the interconnected void spaces, or pores, within the aquifer. Porosity is the ratio of the volume of these void spaces to the total volume of the aquifer material, expressed as a fraction. Because water only travels through these pore spaces, the actual area available for flow is smaller than the total cross-sectional area used in Darcy's Law. Consequently, the actual average velocity of the water particles through these pores, the seepage velocity, must be higher than the discharge velocity. Porosity provides the factor by which the discharge velocity must be adjusted to reflect this. The seepage velocity is calculated by dividing the discharge velocity by the porosity of the aquifer, effectively accounting for the reduced area through which the water actually flows.