Evapotranspiration
Calculation[edit]
Thornthwaite Mather method[edit]
The Thornthwait Mather method of calculating water balance[1] is recommended within the [2003 Ontario Stormwater Management Planning and Design Manual https://www.ontario.ca/document/stormwater-management-planning-and-design-manual]
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)
For monthly calculations, data can be found in Environment Canada Climate Normals
| 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 |
| Day length (hr) | 9 | 10 | 12 | 13 | 15 | 15 | 15 | 14 | 12 | 11 | 9 | 9 |
| Temperature (°C) | - 6.20 | - 4.90 | - 0.30 | 6.90 | 13.30 | 18.70 | 21.40 | 20.30 | 15.90 | 9.10 | 3.10 | - 2.70 |
| Monthly heat index, 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 |
| Corrected PET (mm) | 0.0 | 0.0 | 0.0 | 33.1 | 78.3 | 116.4 | 136.9 | 119.3 | 78.7 | 38.1 | 9.6 | 0.0 |
- ↑ 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.
Thornthwaite1957