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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>


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<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.''' 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>


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