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→Thornthwaite Mather method
We are in the process of writing an xls tool to semi-automate these; please feel free request a draft copy, or to contribute your own [[How to contribute]].
We are in the process of writing an xls tool to semi-automate these; please feel free request a draft copy, or to contribute your own [[How to contribute]].
'''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:
<math>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</math>
'''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>