Hydraulic Head

The methods described herein pertain particularly to the measurement of hydraulic head in soils. Methods are described for measuring hydraulic head both above and below a water table. While the interpretation of hydraulic-head readings is much the same for both cases, the equipment and procedures are quite different. A reference elevation is required as a base for hydraulic-head measurements in any given flow system. It is convenient to select a reference elevation at some depth below the lowest hydraulic-head value in the system. Hydraulic head increases in a positive direction upward from this reference elevation. Logging of the soil materials penetrated provides helpful information for better understanding of ground-water movement and interpreting hydraulic-head readings. Tensiometers and piezometers have a response time that is not zero because of the volume of water that must move into or out of the piezometer or tensiometer to register a pressure change that occurs in the soil water.