Forestry Tasmania Method for estimating Evapotranspiration (ET) ​

ET is an increasing proportion of pan evaporation as temperature decreases, and it continues at temperatures below 8 degrees Celsius (when pan evaporation stops; Graph 1). ET also has a seasonal variation independent of temperature (Graph 2). Thus, for humid vegetated areas calculating ET from pan evaporation underestimates ET during lower temperature periods and overestimates ET during warmer periods.

The relationships in Graph 2, which may not be valid in non-vegetated areas, are:

For background reading, see: Mount, A. B. (1972). The Derivation and Testing of a Soil Dryness Index using Run-Off Data. Forestry Commission Tasmania Bulletin No. 4.