This. Gibraltar built a desalination plant because it was the only option - the Spanish refuse to pump water over the border for political reasons. They relied

This. Gibraltar built a desalination plant because it was the only option - the Spanish refuse to pump water over the border for political reasons. They relied on a moisture condenser built into the side of the rock initially (there is often very moist air caused by the cold air from the atlantic hitting the warm air from the med), but with expansion it wasn't enough and is now used primarily to provide irrigation to the nature reserve.