Evaporimeter, instrument that measures the rate of evaporation of water into the atmosphere, sometimes called an atmometer. Evaporation rates are so sensitive to the water supply, and the nature of the evaporating surface, data collected by Evaporimeter often do not reflect true evaporation processes; hence, Evaporimeter have limited use. The measurement of water evaporation is important in agricultural, hydro-meteorological studies, in the design and operation of reservoirs, and in irrigation and drainage systems. The rate of evaporation is defined as the amount of water lost by evaporation from a unit area of surface in unit time. For a given area this is proportional to the depth of liquid water lost in a day from the whole area and evaporation is usually expressed in this way.