The presence of certain species of vegetation can be a useful indication that ground water or soil moisture lies relatively close to the land surface. These plant indicators are most obvious in arid parts of the world, where green vegetation stands out, but the principle of using plant species as an index to locate ground water near the surface is equally useful in humid countries. The best relationships are found between certain groups of plants (called plant associations) and the depth of ground water or the salinity of water. In North Africa, for example, research has identified various plant associations (usually three to four main species per association) and their relationship to ground water depth and salt content of the water. The presence of certain trees and shrubs, for example the "salt cedar" type trees (Tamarix species), indicates salty water. Similarly, in the arid western U.S., Tamarix species, cottonwood trees, willows and other plants are associated with shallow ground water tables.

Plants whose roots actually tap the ground water are called "phreatophytes." Due to their high transpiration rates in arid zones, the phreatophytes can "pump out" a small stream or lower the level of a well. This transpiration loss could be of concern if, for example, many trees or other deep-rooted plants are planted around a well for shade or to stabilize sand in a dry, windy setting. High transpiration by the plants also can increase the salt concentration in the well water.

In arid zones, the perennial plants, especially trees and shrubs, are the most useful indicators of ground water. Annual plants, mainly legumes and grasses, are generally not good indicators since they come and go depending on rains and the season of the year.

Generally surveys of vegetation to help find shallow ground water are most effective if carried out in the dry season.

It would be useful at this point to present a table of plant species and plant associations, country by country. Unfortunately this information is not available for most countries, at least not in published form. Even if feasible, a list of all the plant species would be much too large. Finally, most people would need plant pictures and descriptions to accompany the names. You will therefore have to make the effort locally to determine the local plants which are good indicators of ground water, Sources of possible information include experienced well diggers or drillers in the area; water resource engineers; in rare cases, published reports (e.g., old FAO reports); and research station or university botanists. In many cases the necessary information can come only from interviews with these local sources of information.