Evapotranspiration

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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
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
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