A piezometer is a tube or sensor that measures groundwater head at one chosen depth, by being open to the ground only over a short interval.
A piezometer measures hydraulic head at a specific depth. The simplest type is a small-diameter standpipe with a short screen at the bottom. The space around the pipe above the screen is sealed so that water can enter only at that depth. The level in the pipe is then read with a water level tape or a pressure sensor. This differs from an ordinary observation well, which may be open over a long section and shows a mix of heads from several layers. Two or more piezometers at different depths in one place, called a nest, show whether water is moving up or down. A downward gradient suggests recharge, and an upward gradient suggests discharge. Other types include vibrating wire and pneumatic piezometers, which are sealed into the ground and read remotely. They respond quickly in clay, where a standpipe would take a long time to fill. Piezometers are used in groundwater monitoring, and also in dams, embankments and tunnels, where high water pressure can threaten stability. Common errors include a poor seal, a clogged screen and readings not tied to a surveyed elevation.