Hi Mostafa,

Air vapor pressure deficit is the difference between saturation and actual vapour pressure for a given time period. Saturation vapour pressure can be computed using maximum and minimum temperature. Actual vapour pressure can be computed using minimum and maximum relative humidity or by using mean humidity and actual vapour pressure. I have attached you the formulas that you need to estimate vapor pressure deficit. 

Mostafa:

   The vapor pressure deficit of the air can be calculated according to the equation proposed by Jones (1992) (equation 1):

VPD = 0.61137et * (1 – UR/100)          (1)

Where:

t is calculated by equation 2:

t = 17,502 * (Tar) / (240.97 + Tar)  (2)

Where:

  Tar = air temperature

Jones HG (1992) Plants and microclimate. A quantitative approach to environmental plant physiology.2. ed. Cambridge: Cambridge University Press.

Might I add that often water vapour pressure defict is computed relative to leaf. Leaf to air differences depend on energy balance and are used to monitor plant caopy stress using Crop water stress index. An intro is possibly http://5e.plantphys.net/article.php?id=42 A more detailed formulation is given by Susan Moran et al. in http://afrsweb.usda.gov/SP2UserFiles/Place/53442010/RemoteSensing/WCLPUB-1735-Moran.pdf

Thanks so much for your useful answers. 

I used the formula by Jones HG. the range of VPD that i computed is between 0.5 and 12 during the year. do you have any idea about the range of VPD in temperate region?my site is located in the Alps.

Hi Masoumeh 

Thanks for contribution.

The VPD depends on air temperature and air relative humidity so VPD in mid day is very variable than it in morning and night however our estimates was between 2 and 10 from 15 April to 30 Jun in mid day at Tehran.

Wish that helps you 

yes, thank you. actually i used the mean daily temperature and mean daily relative humidity for all over the year. comparing with your range my computation sounds logical.

Dear Afrah: 

You can refer FAO 56 in page 40.  http://www.kimberly.uidaho.edu/water/fao56/fao56.pdf

 Photo period depends on latitude so it can be found easily for each region on the internet. 

Dear Dr. Redmond Ramin Shamshiri

Thanks for providing The formula. 

Best regards 

Mostafa

unit of es if kPa

https://physics.stackexchange.com/questions/4343/how-can-i-calculate-vapor-pressure-deficit-from-temperature-and-relative-humidit

Dear Mostafa

Send you an example for the calculus of DPV in excel using data from Antumapu meteorological station, in Santiago, Chile. It is very simple, I hope it will be useful to you

Best regards

LM