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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)?

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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

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