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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

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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

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Thornthwaite equation (method) model results

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

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