Basic concepts are explained in this Chapter with an emphasis on better understanding the different ways in which the energy that drives water flow through the unsaturated and saturated zones can be described. Energy per unit mass or energy per unit weight are both used extensively in various scientific disciplines to describe the water potential. Hydraulic head can be related to both but as studies extend to the consideration of waters of different density mixing, then a more careful analysis is required than use of the simple hydraulic head concept. New equipment is described that will allow the water potential in shallow sand aquifers where mixing of salt water and fresh water occurs – the important coastal fringe.

Measurements of water potential are impacted by changes in the total stress on an aquifer. These changes may be due to diel variation in atmospheric pressure or to earth tides. The advent of data loggers capable of recording many thousands of samples means that new techniques are required to interpret this data, Fourier analysis techniques are reviewed and examples of field data analysis using these techniques is given.