People are naturally going to live near clean water sources, that way it's easier to survive, so people are more likely to live near or in an oasis than in a hot dry area like a desert.
The ultimate source of energy that lifts and transports the atmospheric water (vapor and cloud) to its highest elevations is solar energy. It is the sun that gives enough energy to water for its evaporation.
Religious and ethnical problems.
It was one of the largest empires of the world with many different ethnicities. The problem was to unite people under one rule.
Another problem was communication. It was hard to transfer knowledge and news to all parts of the empire because it was too large.
