Glossary
Pan Evaporation & Pan Coefficient
Pan evaporation is the daily water loss measured from a standardized open pan (such as the US Class A pan). The pan coefficient is a scaling factor (often around 0.7) applied to that reading to estimate evaporation from a larger natural water body.
A small metal pan heats and cools faster than a large reservoir and is more exposed, so it usually over-reads relative to open water. The pan coefficient corrects for this; its value varies with pan type, climate, season, and surrounding conditions.