4 Equipotential Lines and Flow Direction
Having established the concept of hydraulic gradient and the fact that groundwater flows from locations of higher head to regions of lower head, the objective of this section is to introduce how hydraulic gradients manifest in the form of equipotential lines, which are commonly plotted as equipotential contours on maps and in cross section. These, in turn, provide a means to infer the direction and magnitude of hydraulic gradients and flow directions. Understanding how to create and interpret equipotential contour maps and equipotential contour cross sections is essential to successful management of groundwater resources.
This section can serve as an introduction to flow nets. Graphical construction of flow nets is presented in the Groundwater Project book by Poeter and Hsieh (2020) in which the general characteristics of equipotential lines and associated flow directions are introduced for various idealized hydrogeologic conditions. This book discusses hydraulic head profiles as they relate to contours of equipotential as illustrated in cross sections.
The discussion of equipotential lines, flow directions, and gradients presented in this section is valid for isotropic conditions (Kx=Ky=Kz), which means hydraulic conductivity has the same value when measured in any direction. This simplification is not a compromise, rather it enables us to more easily understand the underlying concepts that form the basis for more complex situations. In fact, when analyzing hydrogeologic systems in the field, we often assume isotropy in the initial analysis, and it is not always necessary to go beyond this simplification. This assumption may be loosened to account for anisotropic conditions, in which K is not the same in all directions (Woessner and Poeter, 2020).