Colorado sunken pan
From Glossary of Meteorology
Colorado sunken pan
A type of evaporation pan that is about 1 m (3 ft) square and 0.5 m (18 in.) deep.
This pan is sunk into the ground to within about 5 cm (2 in.) of its rim, and the water is maintained at about ground level. It is made of unpainted galvanized iron. The pan coefficient, on an annual basis, is about 0.8.