The flow of water in soil occurs through interconnected openings between the soil particles. The flow of water through the soil is erratic, and its velocity changes radically in space: the velocity is large in the small pores and small in the larger ones.

We do not need to determine the path that the water particles follow in their way through the soil for most engineering groundwater flow problems. It usually is sufficient to determine average velocities, average flow paths, the discharge flowing through a given area of soil, or the pressure distribution in the soil. We work, throughout this text, with averages and ignore the actual paths of flow. We use the term rectilinear flow, for example,when the average flow is in one direction.

The theory of groundwater flow is based on a law discovered by Henry Darcy [1856]. After the introduction of the basic concepts, we discuss the experiment performed by Darcy and present his law in its simplest form. We then present the generalized form of Darcy's law and the equation of continuity, and finish the chapter by combining these two equations into one governing equation for steady flow of a homogeneous fluid in a porous medium.

Basic Concepts

The basic quantities used to describe groundwater flow are velocity, discharge, pressure, and head. We discuss these quantities next.

The Specific Discharge

We define specific discharge as the volume of water flowing through a unit area of soil per unit time. The units of specific discharge are [L3/(L2T)], or [L/T], and thus are the same as those of a velocity. Specific discharge sometimes is called discharge velocity, but we use the term specific discharge to avoid confusion with a velocity. We represent the specific discharge by q.

The seepage velocity v is the average velocity at a point of the porous medium; it is the specific discharge divided by the area of voids present in a unit area of porous medium. If the porosity of the medium is n, then the area of voids per unit area is n and therefore

Since n is always less than 1, v is always larger than q.