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→Thornthwaite Mather method
where:
where:
T = mean monthly temperature.
T = mean monthly temperature.
Step 2. Calculate the annual heat index (I):
Step 2. Calculate the annual heat index (I):
where: α = 6.75 x 10<sup>-7</sup> ''I<sup>3</sup>'' - 7.71 x 10<sup>-5</sup> ''I<sup>2</sup>''+ 0.0179 ''I'' + 0.49
where: α = 6.75 x 10<sup>-7</sup> ''I<sup>3</sup>'' - 7.71 x 10<sup>-5</sup> ''I<sup>2</sup>''+ 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):
<math>PET=PET_{uncorrected}\cdot\frac{N}{12}\cdot\frac{daylight\ hours}{30}</math>