For many years, measurements taken on evaporation pans have been used
to provide estimates of the amount of evaporation from lakes and reservoirs.
The popularity of pans for this purpose stems from the fact that they are
inexpensive, simple to instrument, and generally, the annual ratio of lake
to pan evaporation remains reasonable constant from year to year, and to
a lesser extent, from region to region.
The Tirtaharapan Differential Pressure Logger mounted on a storage reservoir in combination with a standard surface pan, the U.S. Weather Bureau Class A evaporation pan can be used to measure open water evaporation.
The logger measures the decrease of the water level in the reservoir with respect to the evaporation of the water in the A- pan in time. The pan coefficient is taken equal to the ratio of the lake evaporation to the pan evaporation. A correction for the Advected Energy is suggested by Kohler et al(1955) for shallow lakes.
Evapotranspiration.
Similar designs are possible in combination with Tanks and Lysimeters. The "percolation" is collected in a receiving tank equipped with a Differential Logger.