The Bowen ratio is defined as follows (Ponce, 1989):
in which Q = energy expended in the evaporation process; _{e}T = water surface temperature, in °C; _{s}T = overlying air temperature, in °C;
_{a}e = saturation vapor pressure at the water surface temperature, in millibars (mb); _{s}e = vapor pressure of the overlying air, in mb; _{a}p = atmospheric pressure, in mb;
γ = psychrometric constant, in mb °C^{-1} (Note that 1000 has the units of mb).
The psychrometric constant γ is expressed as follows:
in which ^{-1} °C^{-1}; p = atmospheric pressure, in mb;
λ = latent heat of water vaporization, in cal gr^{-1}; and r = ratio of the molecular weight of water vapor to dry air: _{MW}r = 0.622.
_{MW}
The specific heat of air between 0°C and 40°C is: ^{-1} °C^{-1} = 0.24017 cal gr^{-1} °C^{-1}.
The mean sea-level atmospheric pressure is p = 1013.25 mb. Therefore, at sea level, the psychrometric constant is:
which reduces to:
with λ in cal gr
Since λ varies with temperature, γ also varies with temperature.
Table 1 shows the variation of the psychrometric constant γ with temperature, at standard sea-level atmospheric pressure (
The variation of psychrometric constant γ with temperature has been explained and clarified.
This should result in increased accuracy in online evaporation calculations (
Ponce, V. M. 1989. |

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