I like your method to ask a question. If you want to calculate the evapotranspiration during a season, you have multiple choices. You can find a number of simple methods to do this from the following review paper (It can be downloaded from reseachgate space or directly from webpage of the Journal). Most methods have reasonable auucracy in estimate seasonal evapotranspiration in spite that it is great challenge to estimate its inter-annual or decadal variability.

Wang, K., and R. E. Dickinson (2012), A review of global terrestrial evapotranspiration: Observation, modeling, climatology, and climatic variability, Rev. Geophys., 50, RG2005, doi:10.1029/2011RG000373.

Thanks a lot , i am downloading the paper now

i mean ETo for every crop erea during one season.

is Penman-Monteith equation suitable FOR that?

Penman-Monteith equation is enough. However, it requires a number of input data. If you don't have these input data, some simplified methods may also works.

Thanks a lot

Noha you may go through following paper

Article Estimation of Crop Coefficient and Evapotranspiration of Whe...

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Svetimir DRAGOVIC, Velibor SPALEVIC, Veljko RADOJEVIC, Milinko CICMIL, Milenko USĆCMLIC: IMPORTANCE OF CHEMICAL AND MICROBIOLOGICAL WATER QUALITY FOR IRRIGATION IN ORGANIC FOOD PRODUCTION. Agriculture & Forestry, Vol 55. (09) (1-4): 83-102, Podgorica, 2012

http://www.agricultforest.ac.me/paper.php?journal_id=158&id=2037

Svetimir DRAGOVIC: EFFECT OF IRRIGATION ON FIELD CROPS YIELD UNDER THE VARIABLE AGRO-CLIMATIC CONDITIONS OF SERBIA

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Penman-Monteith is considered as the best estimator for croplands by FAO (Allen et al 2004), provided one has net radiation, windspeed, rel humidity and air temp, as well as crop coefficients, as pointed out by Waldo. If one does not have all the met data, then one can use temp alone (Hamon) or if in a subtropical humid area, net radiation alone (Abtew), or the Turc method. Now, if one is trying to estimate ET for woody vegetation and forests, the problems begin, as it is not possible to accurately measure water uptake by trees (sap flow methods have their own errors), and then water uptake differs vastly by species and phenology. Models attempt to take veg characteristics into account using leaf area index, veg ht etc, but in the end these are just estimates.

thanks alot , i am using Penman-Monteith equation now

http://www.fao.org/docrep/X0490E/X0490E00.htm

Thanks alot already use it :)

but it's useful answer for any one search for penman - monteith equation and show the question

Penman-Monteith ????

@R. Mohebi what's wrong??

i think that the Penman-Monteith is useful to calculate the evaporate . but i want that software.

You may also want to have a look at various methods based on the energy balance approach that integrate information derived from satellite imagery with measured meteorological data at the time of satellite overpass and provide actual daily values. Referent evapotranspiration calculated using FAO Penman Monteith equation is basically an atmospheric water demand as it uses only meteorological data and to relate those values with crop evapotranspiration, crop coefficient must be applied. The approach based on remote sensing takes into consideration not only meteorological data but also surface characteristics (whether it is bare surface or vegetation). Once calculated, daily actual evapotranspiration can be extrapolated to longer time scales (e.g. month, season) through establishing functional relationship with the known parameter(s) that can be assumed to have similar temporal variation as actual evapotranspiration. One of the advantages of such approach is that it enables estimation of spatial variation of ET as well as its quantification. Of course, the care must be taken when it comes to characteristics of the chosen sensor (especially in terms of spatial resolution in comparison with the size of study area).

Use FAO ETo calculator for the purpose. You can also download manual of this software for help.

Thanks a lot

Penman-Monteith gives the potential evapotranspiration for a particular vegetation cover (defined from the stomatal and aerodynamic resistances). Generally, this is for a reference crop (clipped grass is typically used), and a crop coefficient used to convert the reference crop estimate to the vegetation cover you are interested in. Converting to actual evapotranspiration (I assume this is what you are really after) requires additional information: soil moisture and a model of how the actual to potential evapotranspiration depends on the soil moisture. If the soil is sufficiently moist throughout the season (e.g. wet seasons in monsoon areas), the actual ET will equal potential ET, but you do need to be careful to check there were not dry spells within the season.

It is possible to get an estimate of the annual actual evapotranspiration from observations of the rainfall and discharge providing that the soil moisture, groudnwater storage etc are sufficiently similar at the start and end of the year. Going to a seasonal scale, you are not able to use this simple method - you also need information on the differences in the storages from the start to the end of the season.

Www.fao.org/docrep/x0490e/x0490e0c.htm‎

Noha, the question seems unclear!

What do you mean by "during one season"? and what is the difference between one season, one year, and one month?

I understood from your answers and profile that you worked with PM method and models, and you are professional in this topic, so what is the problem with you to calculate ET during one season?

I would like to help you but please clarify your request...

You mean actual or potential evapotranspiration?

It depends what kind of data do you have...... You can use Penman Monteith if you have meteorological data, and if you have some humidity and temperature gradient you can estimate by Bowen Ratio or Eddy Covariance..... To estimate by Bowen Ratio folow instruction attach

Article Temporal Patterns of Energy Balance for a Brazilian Tropical...