Used to measure the evaporation of water.
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.
Evaporation is generally measured at meteorological and agro-meteorological stations using a pan evaporimeter. The values obtained with the pan do not give directly the evaporation from a surface of natural conditions. They indicate, however the order of magnitude of evaporation from such surfaces and comparative results over different areas may be obtained by using identical instruments and similar exposures.
The pan evaporimeter consists of a cylindrical reservoir made from copper sheet of fixed diameter and depth, filled with water to a few centimetres below the rim. A fixed-point gauge in a stilling well serves to indicate the level of water in the pan. A calibrated measuring cylinder is used to add or remove water at each observation to bring the water level to the fixed point. The cross-sectional area of the measuring cylinder is such that, the number of millimetres of water added from the measuring cylinder divided by 100 gives the amount of water in millimetres which has evaporated from the pan during a given interval of time. The reservoir is covered with wire-mesh netting to protect the pan from birds and animals. A thermometer suspended from a mounted clamp to the side of the reservoir, records the temperature of the water in the pan.