http://sciencing.com/calculate-using-rainfall-thiessen-method-6799886.html

http://www.yemenwater.org/wp-content/uploads/2013/03/computation-of-average-rainfall-over-a-basin.pdf

https://theconstructor.org/water-resources/mean-precipitation-calculation-over-an-area/4500/

http://file.scirp.org/Html/5-9402024_43175.htm

Thank you very much for your help 

Okey, thank your very much for your help dear Anders

Mourad

Yes, you can but you can try other methods like krigging for better results.