Difference between revisions of "Evapotranspiration"

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Line 25: Line 25:
!  !! Jan !! Feb !! Mar !! Apr !! May !! June !! July !! Aug !! Sept !! Oct !! Nov !! Dec
!  !! 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
! 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
! Day length (hr) || 9 || 10 || 12 || 13 || 15 || 15 || 15 || 14 || 12 || 11 || 9 || 9
Line 31: Line 32:
! 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
! 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
! 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
! 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

Revision as of 16:58, 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
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