Dear Hussein,

I suggest you the following interesting papers.

http://www.adv-geosci.net/11/37/2007/adgeo-11-37-2007.pdf

http://folk.uio.no/chongyux/papers_SCI/JGS_2.pdf

http://onlinelibrary.wiley.com/doi/10.1029/2007WR006563/full

http://cdn.intechopen.com/pdfs/26111/InTech-Critical_review_of_methods_for_the_estimation_of_actual_evapotranspiration_in_hydrological_models.pdf

http://www.whycos.org/hwrp/guide/chapters/english/original/WMO168_Ed2008_Vol_I_Ch4_Up2008_en.pdf

http://www.tandfonline.com/doi/pdf/10.1080/02626668809491261

With my best regards

Prof. Bachir ACHOUR

Dear prof. Bachir,

Many thanks for your answer 

there are several methods that can be listed as follow:

1- look for weather stations sites  on the web until you find sites that include your location.

2- you will find the precipitation,  temperature, humidity, pressure,...etc for your site.

3- use one of the following equations:

Catchment water balance

Evapotranspiration may be estimated by creating an equation of the water balance of a drainage basin. The equation balances the change in water stored within the basin (S) with inputs and outgoes:

The input is precipitation (P) and the outgoes are evapotranspiration (which is to be estimated), streamflow (Q), and groundwater recharge (D). If the change in storage, precipitation, streamflow, and groundwater recharge are all estimated, the missing flux, ET, can be estimated by rearranging the above equation as follows:

Hydrometeorological equations

The most general and widely used equation for calculating reference ET is the Penman equation. The Penman-Monteith variation is recommended by the Food and Agriculture Organization and the American Society of Civil Engineers.The simpler Blaney-Criddle equation was popular in the Western United States for many years but it is not as accurate in regions with higher humidities. Other solutions used includes Makkink, which is simple but must be calibrated to a specific location, and Hargreaves. To convert the reference evapotranspiration to actual crop evapotranspiration, a crop coefficient and a stress coefficient must be used. Crop coefficients referred to in many hydrological models are themselves during periods for which the model is used. This is because crops are seasonal, perennial plants mature over multiple seasons, and stress responses can significantly depend upon many aspects of plant condition.

Energy balance[edit]

A third methodology to estimate the actual evapotranspiration is the use of the energy balance.

where λE is the energy needed to change the phase of water from liquid to gas, Rn is the net radiation, G is the soil heat flux and H is the sensible heat flux. Using instruments like a scintillometer, soil heat flux plates or radiation meters, the components of the energy balance can be calculated and the energy available for actual evapotranspiration can be solved.

The SEBAL and METRIC algorithms solve the energy balance at the earth's surface using satellite imagery. This allows for both actual and potential evapotranspiration to be calculated on a pixel-by-pixel basis. Evapotranspiration is a key indicator for water management and irrigation performance. SEBAL and METRIC can map these key indicators in time and space, for days, weeks.

Khaled_Kheireldin

Dear Mostafa

many thanks 

In reference to equations such as Penman, use caution as many of these do not consider soil moisture (or any other factor that indicates water availability).  Since you indicate arid regions, many of these equations will not give accurate results (http://onlinelibrary.wiley.com/doi/10.1029/2005WR004541/abstract).  Water balance and energy balance may get closer.  Of course, if you have the means, eddy covariance or Bowen ratio towers would be great.

Dear Kahler,

Appreciating your kind answer