Evapotranspiration

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Thornthwaite Mather method[edit]

Step 1. Calculate the monthly heat index (i): where: T = mean monthly temperature.

Step 2. Calculate the annual heat index (I):

Step 3. Calculate an uncorrected Potential Evapotranspiration (PETuncorrected):

where: α = 6.75 x 10-7 I3 - 7.71 x 10-5 I2+ 0.0179 I + 0.49

Step 4. A monthly correction is then applied to account for varying daylight hours and number of days in a month, to find the corrected potential evapotranspiration (PET):