ABSTRACT
This chapter is intended as a guide to the laws governing groundwater flow through the porous subsurface. These laws relate the kinematics of the water “particles”—the time-dependent motion of the “particles” through three-dimensional space—to the dynamics—the forces causing groundwater flow. The fundamental principles are conservation of mass and conservation of momentum. These principles lead to mathematical equations—generally, partial differential equations—in three-dimensional space and time; in 4-D space–time, so to speak. It is important to keep the 4-D “picture” underlying the fundamental equations in mind when working on practical groundwater problems. However, the equations can often be simplified by reducing them to two- or even one-dimensional in space, sometimes also neglecting the time-dependent term.
