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Potential evaporation

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Potential evaporation

Potential evaporation or potential evapotranspiration (PET) is defined as the amount of evaporation that would occur if a sufficient water source were available. If the actual evapotranspiration is considered the net result of atmospheric demand for moisture from a surface and the ability of the surface to supply moisture, then PET is a measure of the demand side. Surface and air temperatures, insolation, and wind all affect this. A dryland is a place where annual potential evaporation exceeds annual precipitation.

Thornthwaite equation (1948)

P E T = 16 ( L 12 ) ( N 30 ) ( 10 T a I ) α

Where

P E T is the estimated potential evaporation (mm/month)

T a is the average daily temperature (degrees Celsius; if this is negative, use 0 ) of the month being calculated

N is the number of days in the month being calculated

L is the average day length (hours) of the month being calculated

α = ( 6.75 × 10 − 7 ) I 3 − ( 7.71 × 10 − 5 ) I 2 + ( 1.792 × 10 − 2 ) I + 0.49239

I = ∑ i = 1 12 ( T a i 5 ) 1.514 is a heat index which depends on the 12 monthly mean temperatures T a i .

Somewhat modified forms of this equation appear in later publications (1955 and 1957) by Thornthwaite and Mather.

References

Potential evaporation Wikipedia
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