Difference between revisions of "Evapotranspiration"
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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}= | + | <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 | ||
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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 | ||
| + | |} | ||
Revision as of 16:55, 8 January 2018
Calculation[edit]
Thornthwaite Mather method[edit]
Step 1. Calculate the monthly heat index (i): where: T = mean monthly temperature.
Step 2. Calculate the annual heat index (I):
Step 3. Calculate an uncorrected Potential Evapotranspiration (PETuncorrected):
where: α = 6.75 x 10-7 I3 - 7.71 x 10-5 I2+ 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):
=Example[edit]
For the town of Richmond Hill (Lat = 43.87)
| 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 |