A pan coefficient is the factor that converts evaporation measured in a pan to the evaporation expected from a lake or reference crop.
A pan coefficient, Kp, converts the evaporation measured in an evaporation pan to the evaporation from a real water surface or crop. It is needed because a small metal pan loses water faster than a lake. The pan absorbs heat through its sides and base, and it has no large mass of water to store heat. For lake evaporation, the Class A pan coefficient is commonly taken as about 0.7 on an annual basis. Monthly values can differ a lot, since a deep lake stores heat in spring and releases it in autumn. For reference crop evapotranspiration, FAO Irrigation and Drainage Paper 56 uses the formula ETo = Kp × Epan. It lists Class A values from about 0.35 to 0.85, depending on humidity, wind speed and whether the pan is surrounded by green crop or dry fallow land. Low humidity and strong wind give lower coefficients. The pan type, its siting and its maintenance all affect the result, so a value from another site should be used with care. Where possible, Kp should be checked against a local water budget or the Penman-Monteith method.