Thornthwaite equation to estimate potential evaporation (PET)

Model : Thornthwaite
Simile version : 3.1+
Date added : 2003-09-06
Keywords : Evapotranspiration ;

Description

Model to calculate potential evaporation (PET) using the Thornthwaite method. Thornthwaite (1944) defines potential evaporation as “the water loss which will occur if at no time there is a deficiency of water in the soil for use of vegetation.” — assuming soil storage is not depleted.

Input files and results for Leuchars, Scotland 56 degrees N, and Boston, USA 54 degrees N, are supplied.

Files

Model file Click on the icon to download the model file. (You will need Simile to examine and run the model. A free evaluation version is available from the products page.) Some browsers may attempt to display the model file, rather than open it in Simile; in this case, use the browser back button to return to this page, and use the context menu (invoked by right-clicking on the link) to save the target file to disk.

Thornthwaite.sml

Diagram

Equations

Potential evapo-transpiration (mm/day),
   PET = if Ta > 0 then dl*16*(10*Ta/I)^a
   else 0                 
where
   Ta is mean monthly temperature (Celsius)
   a = 0.49+0.0179*I-7.71*10^ -5*I^2+6.75*10^ -7*I^3 
   dl = daylength in hours / 12
   I = sum(i)
   i is a monthly heat index given by
     i = if Ta>0  then (Ta/5)^1.5
         else 0         

	

Results

PET, Ta and dl for Boston, USA (in blue) and Leuchars, Scotland (in red).

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