Hydraulic head is a measurement of the amount of energy available in groundwater due to pressures in a water table or the height of the water level in the ground. Often called head, the measurement is shown in feet or meters of water height, normally referenced against sea level to give a standard measurement. A piezometer, which is a small-diameter pipe open only at the top and bottom, can be used to measure the height of water resulting from the hydraulic head in a water table. These measurements can be used to determine available water for pumping operations, or to measure and chart the flow of water in underground supplies.
There are two kinds of underground water supplies. An unconfined water table connects to a lake, stream or other water source at ground level. Water can freely move between the lake and the underground water table based on rainfall or water use. Confined water tables are layers of water-bearing sands or rock contained between solid rock layers, and are not connected to surface water supplies directly. Wells are often drilled to confined water tables, because they may not be as easily disrupted by drought or loss of surface water and can provide more consistent water supplies.
Providing a well water supply requires an understanding of the underground water tables, available hydraulic head, and available flow rates. Companies will first place piezometers at selected locations that confirm the availability of water and allow for hydraulic head measurements. When the piezometer survey is completed, well locations can be chosen and drilled to the depths needed to provide the best hydraulic head. The water table pressure can be important, because low pressure requires larger pumps to provide adequate lifting force to obtain water. A final test is to operate the well at maximum flow rate for a given period of time, which confirms that the water table accessed by the well can provide adequate water.
Water tables or aquifers can vary greatly from season to season, partly due to differing amounts of rainfall. A greater effect on groundwater supplies are the effects of humans, primarily caused by agriculture or farming. In some parts of the world, growing crops requires very large amounts of water, and surface water supplies are often not available or dependable for consistent crop quality. Wells provide a more consistent supply, but large water demands can deplete or empty underground water tables that cannot be recharged quickly.