Evapotranspiration

Calculation[edit]

Thornthwaite Mather method ^[1][edit]

Step 1. Calculate the monthly heat index (i): $i=\left({\frac {T}{5}}\right)^{1.514}$ where: T = mean monthly temperature.

Step 2. Calculate the annual heat index (I): $I=\sum _{i=1}^{12}i$

Step 3. Calculate an uncorrected Potential Evapotranspiration (PET_uncorrected): $PET_{uncorrected}=16\cdot \left({\frac {10\cdot T}{I}}\right)^{\alpha }$

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): $PET=PET_{uncorrected}\cdot {\frac {N}{12}}\cdot {\frac {daylight\ hours}{30}}$

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

↑ 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.File:Thornthwaite1957.pdf

[1] 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.File:Thornthwaite1957.pdf

[1]

Evapotranspiration

Calculation[edit]

Thornthwaite Mather method [1][edit]

Example[edit]

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Thornthwaite Mather method ^[1][edit]