People living in arid regions outside of the Nile Valley primarily rely on oases and aquifers for their water supply. These natural sources provide critical water resources in otherwise dry environments. Desalination is also an option in some coastal areas, where salt is removed from seawater to create fresh water. However, the rain-shadow effect does not typically result in heavy rains in these regions; instead, it leads to arid conditions on the leeward side of mountain ranges.
So, the most accurate response is: The oases and aquifers provide water.