Lysimeter is used to measure crop evapotranspiration where the crop grows in isolated tanks filled with either disturbed or undisturbed soil. Water input and water output is measured; by water balance method and then crop evapotranspiration is calculated. In precision weighing lysimeters, where the water loss is directly measured by the change of water, evapotranspiration can be obtained with higher accuracy and hourly calculation of ETc is possible.
Weighing lysimeters measure crop water used by measuring the change in mass of an isolated volume of soil. While irrigation and precipitation add water and increase the weight of the soil volume, drainage and ET removes water therefore decrease lysimeter weight.
Weighing type lysimeter have been used for many years to measure and study water use, to calibrate reference ET methods for a local area, and to develop crop-coefficient functions for specific crops. Lysimeters of many different designs, sizes, shapes, and measurement systems (analog or digital) have been built over the years.
Digital weighing type lysimeter contain mechanical mechanisms and electrical circuitry to obtain relatively high-resolution measurements. Advances in electronics, data logging equipment and strain-gage load cells have allowed for the design of simple digital weighing lysimeter which is less expensive, accurate and reliable, and require less maintenance. Digital weighing type lysimeter consists of five main components; an inner tank, outer tank, load cell assemblies, drain system, and data logger. The inner tank contains a volume of soil and vegetation isolated from the field. The outer tank isolates the inner tank from the field and provides a foundation for mounting the load cell assemblies. The load cell assemblies supports and monitors the weight of the inner tank. The drain system allows for any excess water accumulated in the inner tank to be removed. Data logger records the weight of soil water and drained water daily and also records the soil moisture and soil temperature data.