<p>Yes.</p>
<p>Evapotranspiration (<span class="math-container">$ET$</span>) can be calculated from latent heat (<span class="math-container">$LE$</span>) by dividing by latent heat of vaporization (<span class="math-container">$\lambda$</span>)</p>
<p><span class="math-container">$$ ET = \frac{LE}{\lambda} $$</span></p>
<p>This assumes density of water equal to 1000 kg/m<sup>3</sup>. Latent heat of vaporization is the amount of energy needed to change a unit mass of water
from liquid to water vapor. With LE and λ in units of MJ m<sup>-2</sup> T<sup>-1</sup> and MJ kg<sup>-1</sup>, respectively, ET is in units of kg m<sup>-2</sup> T<sup>-1</sup> which is equal to depth of water in mm T<sup>-1</sup>.</p>
<p>Latent heat of vaporization varies slightly with temperature, but is often set to 2.45 MJ kg<sup>-1</sup> which is the value for an air temperature of 20 °C. Allen et al. (1998) also provides an equation for calculating λ with air temperature variation, see Annex 3 equation 3-1:</p>
<p><span class="math-container">$$ \lambda = 2.501 - (2.361\times10^{-3})\times T_a$$</span></p>
<p>where <span class="math-container">$T_a$</span> is air temperature in °C and <span class="math-container">$\lambda$</span> is in MJ kg<sup>-1</sup>.</p>
<p>Allen, R. G., Pereira, L. S., Raes, D., &amp; Smith, M. (1998). Crop evapotranspiration - Guidelines for computing crop water requirements. FAO Irrigation and drainage  Paper 56. <a href="http://www.fao.org/3/x0490e/x0490e00.htm" rel="noreferrer">http://www.fao.org/3/x0490e/x0490e00.htm</a></p>

CC BY-SA 4.0
2021-10-09T11:31:37.647Z
2021-10-11T09:27:29.790Z
2021-10-11T09:27:29.790Z
rhkarls
<p>Can we convert latent heat flux to actual evapotranspiration using simply this conversion rate: 2.45 MJ m<sup>-2</sup> day<sup>-1</sup> = 1 mm day<sup>-1</sup> (Allen, 1998)?</p>

<evaporation><energy-balance>
FLUXNET15 - how to convert latent heat flux to actual evapotranspiration?
Pedro Alencar
# FLUXNET15 - how to convert latent heat flux to actual evapotranspiration?

(Asked by: username_0 on 2021-01-28)

Can we convert latent heat flux to actual evapotranspiration using simply this conversion rate: 2.45 MJ m-2 day-1 = 1 mm day-1 (Allen, 1998)?

**Tags:** evaporation, energy-balance


---

## 1 answers

### Answer by username_1 on 2021-10-09. Score: 12 | Accepted answer

Yes.

Evapotranspiration ($ET$) can be calculated from latent heat ($LE$) by dividing by latent heat of vaporization ($\lambda$)

$$ ET = \frac{LE}{\lambda} $$

This assumes density of water equal to 1000 kg/m3. Latent heat of vaporization is the amount of energy needed to change a unit mass of water
from liquid to water vapor. With LE and λ in units of MJ m-2 T-1 and MJ kg-1, respectively, ET is in units of kg m-2 T-1 which is equal to depth of water in mm T-1.

Latent heat of vaporization varies slightly with temperature, but is often set to 2.45 MJ kg-1 which is the value for an air temperature of 20 °C. Allen et al. (1998) also provides an equation for calculating λ with air temperature variation, see Annex 3 equation 3-1:

$$ \lambda = 2.501 - (2.361\times10^{-3})\times T\_a$$

where $T\_a$ is air temperature in °C and $\lambda$ is in MJ kg-1.

Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration - Guidelines for computing crop water requirements. FAO Irrigation and drainage Paper 56. <http://www.fao.org/3/x0490e/x0490e00.htm>


---

