Gs = E / VPD

VPD =  (1 - RH/100) * (Pw,s/Pa)

where VPD is the vapor pressure deficit, RH is the relative humidity, Pw,s is the saturation vapor pressure in the air given temperature in kPa, Pa is the air pressure in kPa. Example at 25 degrees celsius and sea level:

VPD = (1 - 60/100) * (3.169 kPa / 101.325 kPa) = 0.01251

Gs = 10 mmol m-2 s-1 / 0.01251 = 799.36 mmol m-2 s-1

There are a number of formulations to determine gs. The most common is to assume that the temperature of the leaf and the cuvette is the same. Then we can compute rs (stomatal resistance given in cm s-1) (gs = 1 / rs) as follows: (((1 / HR) - 1) / v) - ra, where RH is the relative humidity, A is leaf area exposed in the cuvette, v is the intensity of the air flow and ra is the cuvette resistance, which is specific to each cuvette and you can calculate it placing a wet filter paper in the cuvette instead of the leaf, nullifying rs (rs = 0).

If temperatures (air and leaf) are different, then use the following formulation rs = 1 / gs = ((esh - eo) A / (Eo - Ei) v) - ra where esh is the saturation vapor pressure, eo is the vapor pressure of the cuvette and ei is the vapor pressure of the air entering to the cuvette. If you used a drying agent to dry the air entering the cuvette you can make claculations without much error (1%) with eo = 0. gs  (mm s-1) = gs (mol m-2 s-1) = (273 / (T + 273)) (P / 101.3), where T is the air temperature and p is the atmospheric pressure (You can use the site averaged p where measurements were performed. I hope this formulations are for your calculations. Cheers!

Thank you, Víctor!

Very useful. my best wishes!

thank you!

http://cdn.intechweb.org/pdfs/26114.pdf

thanks

THANK YOU

Gs = E / VPD

Please tell me how can we calculate the value of E in the above equation..?

E is the evapotranspiration rate, measured by the IRGA.