Groundwater flows in response to differences in mechanical energy within the saturated zone. Generally, groundwater moves from points of high mechanical energy to points of low mechanical energy. The difference in mechanical energy between two points divided by the distance between those points is called an energy gradient. In general, groundwater flows in the direction of the steepest energy gradient. (This rule does not apply to anisotropic aquifers, covered later in this chapter.)

Recall from Chapter 2 that hydraulic head expresses the mechanical energy of groundwater. In order to conceptualize groundwater flow at a field site, one must determine the three-dimensional distribution of hydraulic head. Keep in mind that while paper diagrams often depict groundwater flow in two dimensions, as in a map view or cross section perspective, groundwater flows in three-dimensional space, and the direction and magnitude of flow at any given location varies over time.