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→Thornthwaite Mather method
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}=1.6\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>
===Example==
For the town of Richmond Hill (Lat = 43.87)
{| class="wikitable"
! !! Jan !! Feb !! Mar !! Apr !! May !! June !! July !! Aug !! Sept !! Oct !! Nov !! Dec
|-
| Declination (rad) || -0.372 || -0.232 || -0.035 || 0.171 || 0.330 || 0.407 || 0.372 || 0.239 || 0.052 || -0.157 || -0.325 || -0.407
|-
| DayLength (hr) || 9 || 10 || 12 || 13 || 15 || 15 || 15 || 14 || 12 || 11 || 9 || 9
|-
| T || -6.20 || -4.90 || -0.30 || 6.90 || 13.30 || 18.70 || 21.40 || 20.30 || 15.90 || 9.10 || 3.10 || -2.70
|-
| i || 0.00 || 0.00 || 0.00 || 1.63 || 4.40 || 7.37 || 9.04 || 8.34 || 5.76 || 2.48 || 0.48 || 0.00
|-
| PET (mm, uncorrected) || 0.00 || 0.00 || 0.00 || 29.91 || 62.44 || 91.50 || 106.44 || 100.33 || 76.28 || 40.80 || 12.19 || 0.00
|-
| days in month || 31 || 28 || 31 || 30 || 31 || 30 || 31 || 31 || 30 || 31 || 30 || 31
|-
| Monthly PET (mm, corrected) || 0.0 || 0.0 || 0.0 || 33.1 || 78.3 || 116.4 || 136.9 || 119.3 || 78.7 || 38.1 || 9.6 || 0.0
|}