An area in a desert where there is water and trees can grow -

Oasis (noun): 
English Meaning - An area in a desert where there is water and trees can grow. 
Bangla Meaning - মরূদ্যান। 
Example Sentence - I do not want the desert to swallow up the oases.

Other options, 
Arid (অনুর্বর) - Very dry and without enough rain for plants. 
Savannah (সাভানা) - A large, flat area of land covered with grass, usually with few trees, that is found in hot countries, especially in Africa. 
Mirage (মরীচিকা) - An image, produced by very hot air, of something that seems to be far away but does not really exist. 

Source: Cambridge Dictionary.