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→Thornthwaite Mather method
<math>i=\left ( \frac{T}{5} \right )^{1.514}</math>
<math>i=\left ( \frac{T}{5} \right )^{1.514}</math>
where:
where:
''T'' = mean monthly temperature.
''T'' = mean monthly temperature (°C).
'''Step 2.''' Calculate the annual heat index (''I''):
'''Step 2.''' Calculate the annual heat index (''I''):
<math>I=\sum_{i=1}^{12}i</math>
<math>I=\sum_{i=1}^{12}i</math>
'''Step 3.''' Calculate an uncorrected Potential Evapotranspiration (''PET<sub>uncorrected</sub>''):
'''Step 3.''' Calculate an uncorrected Potential Evapotranspiration (''PET<sub>uncorrected</sub>'', mm):
<math>PET_{uncorrected}=16\cdot \left ( \frac{10\cdot T}{I} \right )^{\alpha }</math>
<math>PET_{uncorrected}=16\cdot \left ( \frac{10\cdot T}{I} \right )^{\alpha }</math>
<math>\alpha=6.75\times10^{-7}I^{3}-7.71\times10^{-5}I^{2}+0.0179I+0.49</math>
<math>\alpha=6.75\times10^{-7}I^{3}-7.71\times10^{-5}I^{2}+0.0179I+0.49</math>
'''Step 4.''' Apply a monthly correction to account for varying daylight hours and number of days in a month, to find the corrected potential evapotranspiration (''PET''):
'''Step 4.''' Apply a monthly correction to account for varying daylight hours and number of days in a month, to find the corrected potential evapotranspiration (''PET'', mm):
<math>PET=PET_{uncorrected}\cdot\frac{N}{12}\cdot\frac{daylight\ hours}{30}</math>
<math>PET=PET_{uncorrected}\cdot\frac{N}{12}\cdot\frac{daylight\ hours}{30}</math>