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→Thornthwaite Mather method Thornthwaite, C. W., and Mather, J. R., Instructions and Tables for Computing Potential Evapotranspiration and the Water Balance, Drexel Institute of Technology, Laboratory of Climatology, 1957.{{#l:Thornthwaite1957.pdf}}
==Calculation==
==Calculation==
===Thornthwaite Mather method <ref>Thornthwaite, C. W., and Mather, J. R., Instructions and Tables for Computing Potential Evapotranspiration and the Water Balance, Drexel Institute of Technology, Laboratory of Climatology, 1957.{{#l:Thornthwaite1957.pdf}}</ref>===
===Thornthwaite Mather method===<ref>Thornthwaite, C. W., and Mather, J. R., Instructions and Tables for Computing Potential Evapotranspiration and the Water Balance, Drexel Institute of Technology, Laboratory of Climatology, 1957.{{#l:Thornthwaite1957.pdf}}</ref>
Step 1. Calculate the monthly heat index (i):
'''Step 1.''' Calculate the monthly heat index (i):
<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.
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>):
<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>
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):
'''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>
<math>PET=PET_{uncorrected}\cdot\frac{N}{12}\cdot\frac{daylight\ hours}{30}</math>