The rate of evaporation is defined as the amount of water evaporated from a unit surface area per unit of time. This parameter is an important indicator of the need for water of agricultural area's and also for hydrologists to monitor the water balance of an area. In agriculture people rather talk of evapotranspiration, meaning the combined evaporation of the soil and the transpiration of plants.

Although there are different instruments for performing a direct measurement of the evaporation, the reality is that most evaporation figures are based on a the measurement of a complete set of meteorological parameters. An example of this is the Campbell Scientific or ELE Bowen Ratio System.

A specialist for evaporation gauges is Novalynx.

Atmometers are instruments that measure the loss of water from a wetted porous surface. A current manufacturer is ET gage company. Evaporation pans and tanks are usually filled with water and are open on the surface. Every day the pan is refilled and the water level is read from a scale. Manufacturers are Thies and Lastem.

The problem with pans is that they are not representative of the area and surface structure. In order to make a measurement that takes these factors into account, it is possible to physically isolate a portion of land from its surroundings and continuously weigh it to monitor changes in water content. This is called a lysimeter, also called evapotranspirimeters. Many scientific institutes have lysimeters in operation.

Suppliers are not known to us. Suggestions are most welcome.