Illinois State Water Survey - University of Illinois at Urbana-Champaign

State Climatologist Office for Illinois

Pan Evaporation Across Illinois

Dr. Jim Angel, State Climatologist

Pan evaporation is measured using a four-foot diameter Class A evaporation pan at a few sites across Illinois during the warm season. Measurements are not made in colder months over concern that the water could freeze and damage the pan. The units are in inches of water lost per month. The average is based on all available data rather than a standard period due to the sparseness of data.

The pan evaporation data are now available as a Microsoft Excel spreadsheet and as a web page generated by Excel. Data are updated through 2014, if available.

Evaporation from a natural body of water is usually at a lower rate. Most textbooks suggest multiplying the pan evaporation by 0.75 to correct for this. Data Disclaimer.