General:
* Dry
* Thirsty
* Arid
* Dehydrated
* Scorched
* Baked
* Desiccated
More specific:
* Cracked (for skin)
* Dusty (for ground)
* Barren (for land)
* Withered (for plants)
* Sunburnt (for skin)
The best synonym will depend on what you are trying to convey. For example, "dry" is a general synonym, while "scorched" implies something has been damaged by heat.