From AMS Glossary
A type of evaporation gauge that consists of a tank or pan of soil placed in a field and manipulated so that the soil, water, thermal, and vegetative properties in the tank duplicate as closely as possible the properties of the surrounding area.
Relatively sophisticated lysimeters use various methods to determine the reductions in weight of the instruments so that water loss due to evapotranspiration can be computed. In some cases, the water table level in the lysimeter is adjusted to coincide with a high water table outside the tank. Lysimeters are used for several other purposes. In a potential evapotranspirometer, the water table in the tank is held sufficiently high so that vegetative evapotranspiration is not limited by soil moisture stress. Some types of lysimeters are devoted to studies of chemical leaching by water or to the fate of nutrients or potentially toxic substances, for which water is removed from near the bottom of the tank for chemical analysis. Shear-stress lysimeters have been developed to measure the surface momentum flux from the atmosphere by use of strain gauges or displacement meters attached to the tank.