I think it depends on the nature of the irrigation area served by the drain itself, whereas the raw drain water that is not contaminated with domestic pollutants is preferable for re-watering than the contaminated one.
I agree with you Mr Hommadi, Iraq main outfall drain especially northen can play a good role in this field of management. It has Drainage water may be considered not high salinity due to mixing with irrigation water releases by irrigation escape structures which discharge its water to it.