Dear Vikram Kumar,

The following links will be useful to you.

https://www.researchgate.net/publication/256979768_Estimating_Actual_Evapotranspiration_Using_RS_and_GIS

http://ac.els-cdn.com/S1125786513000787/1-s2.0-S1125786513000787-main.pdf?_tid=a38fa4fc-5057-11e6-9ac9-00000aacb35d&acdnat=1469224981_7c1d0d0a221a97236da80eab3e7c9047

http://scialert.net/qredirect.php?doi=ijar.2015.65.73&linkid=pdf

http://download.springer.com/static/pdf/634/art%253A10.1007%252Fs11269-015-1037-4.pdf?originUrl=http%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2Fs11269-015-1037-4&token2=exp=1469225832~acl=%2Fstatic%2Fpdf%2F634%2Fart%25253A10.1007%25252Fs11269-015-1037-4.pdf%3ForiginUrl%3Dhttp%253A%252F%252Flink.springer.com%252Farticle%252F10.1007%252Fs11269-015-1037-4*~hmac=986617d5c9c2811baa75abe5cd7ee86270f0a22242bc2e3ec98c043c33bbee24

I hope it helps.

All the best !

Article Estimating Actual Evapotranspiration Using RS and GIS

Dear Kumar,

If you have only meteorological data and you want to quantify crop water requirement, you need to estimate de reference ET for which the best well known method is the FAO Penman-Monteith equation described here:

http://www.fao.org/docrep/X0490E/x0490e00.htm#Contents

But if you want to calculate the actual evapotranspiration you must use other method as the mass balance method using lysimeters, or energy balance methods such as Bowen Ratio or Eddy Covariance methods. These energy balance methods uses meteorological data but they need to be measured by specific, non-conventional, weather stations.

I hope I have helped... Feel free to ask further information if you wish..

Best regards.

 @Fernando: Thanks for your help and sharing valuable information. 

I just have to quantify the water requirement for their agriculture practices. So I think Penman-Monteith is good one but at same time I wanted to compare the estimated ET by other methods also. I just wanted to check the better suitability of method for Himalaya region.

Dear Vikram,

                          there are three methods for this that i know, Penman-Monteith, Priestley-Taylor and Hargreaves method.

these three methods you can use for comparasion.