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The Bowen ratio is the ratio between sensible heat and latent heat, which is equal to heat loss or heat gain. This ratio is used to calculate the type of heat transfer for a moist surface. Heat transfer occurs as sensible heat or latent heat.
Sensible heat: This is the difference in temperature without evaporation or change in temperature with no change in phase.
Latent heat: It is the energy required during a change of state at constant temperature.
The Bowen ratio is denoted by BO.
i.e.BO = Qh/Qe
Where Qh is sensible heat and Qe is latent heat.
When BO is less than 1 then latent heat has a greater proportion of energy than sensible heat.
When BO is greater than 1 then sensible heat has a greater proportion of available energy then latent heat.
When BO is equal to zero then the Bowen ratio becomes unbounded. For this reason the evaporation fraction is a more appropriate choice of variable.
The Bowen ratio is related to the evaporative fraction EF and the equation is:
EF = Qe/(Qe+Qh) = 1/(1+BO)
It is an indicator of type of surface and BO is less than 1 over surfaces with an abundant water supply. For example:
Type of surface | Range of Bowen ratio |
Desert | >10.0 |
Semi-arid landscape | 2.0 – 6.0 |
Temperate forest and grassland | 0.4 – 0.8 |
Tropical rainforest | 0.1 – 0.3 |
Tropical ocean | < 0.1 |
Equilibrium Bowen ratio
Equilibrium Bowen ratio is the ratio under equilibrium conditions when the air is saturated. This ratio is large at low temperatures, for example at high altitudes. And it is very low in the tropics, assuming the surface is wet and the air is not too dry, for example over the ocean.
The Bowen ratio approaches the value Be , which is the equilibrium Bowen ratio when the surface and the air at the reference level are saturated.
I.e. BO = Qh/Qe =Be
We know that the actual Bowen ratio is smaller than the equilibrium Bowen ratio over wet surfaces because the air at reference height is not saturated. As a result of the strong temperature dependence of saturation vapour pressure, latent cooling is more than sensible cooling of the surface; but in high latitudes, during winter, sensible heat can be of the greatest importance. For this, assume a wet surface and saturated atmosphere. So that actual Bowen ratio is the equilibrium Bowen ratio.
I.e. Qe/(Qe+Qh) = 1/(1+Be)
Be = 1 when the temperature is 278K.
Be << 1 when the temperature is warmer and the surface is cooling by evaporation.
Be >> 1 when the temperature is colder and the surface is cooling by sensible heat transfer. This condition only applies where the surface is wet.
Evapotranspiration: How good is the Bowen ratio method?
Evapotranspiration is the loss of water to the atmosphere. For calculating evapotranspiration, we use the Bowen ratio method.
The Bowen ratio method approach to determining evapotranspiration from plant communities can give good results under moist conditions, but this method may not be so accurate under very dry conditions.
Conclusion
The Bowen ratio is the ratio between sensible heat and latent heat, and gives the value of heat loss or gain. The equilibrium Bowen ratio is equal to the Bowen ratio under some conditions, which are mentioned above.
We have also learned that a negative Bowen ratio occurs when sensible heat flux is downward and latent heat flux is upward. The Bowen ratio also depends on the type of surfaces, i.e. different surfaces have different Bowen ratios. The Bown ratio is helpful in the field of agriculture, especially in agricultural engineering.
