From AMS Glossary
A circular evaporation pan, six feet in diameter and two feet deep, made of unpainted galvanized iron.
The pan is buried in the ground so that about two inches of the rim extend above the surrounding ground, and the water level is maintained at about ground level. This installation reduces the temperature variations in the pan and causes its pan coefficient to be nearer unity than that of exposed pans. The average pan coefficient is reported to be about 0.9. The initials BPI stand for Bureau of Plant Industry (U.S. Department of Agriculture), which first introduced this instrument.