Evapotranspiration
Calculation[edit]
Thornthwaite Mather method[edit]
Step 1. Calculate the monthly heat index (i): Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle i=\left ( \frac{T}{5} \right )^{1.514}} where: T = mean monthly temperature.
Step 2. Calculate the annual heat index (I): Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle I=\sum_{i=1}^{12}i}
Step 3. Calculate an uncorrected Potential Evapotranspiration (PETuncorrected): Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle PET_{uncorrected}=16\cdot \left ( \frac{10\cdot T}{I} \right )^{\alpha }}
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): Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle PET=PET_{uncorrected}\cdot\frac{N}{12}\cdot\frac{daylight\ hours}{30}}
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 |
| 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 |