* Dehydrate: This refers to removing water, the opposite of supplying water.
* Dry: This is a more general term meaning lacking moisture.
* Desiccate: This refers to the process of drying something completely, often by removing all moisture.
* Wither: This describes the process of becoming dry and shriveled, often due to a lack of water.
The best antonym for "irrigate" will depend on the specific situation you are considering.