Define the mandate of heaven. how does a ruler earn it? how and why is it lost?

The Mandate of Heaven is an ancient Chinese belief that heaven granted rulers the right to rule. A ruler was granted the right to rule based on his ability to govern well and fairly.  A ruler or emperor lost the right to rule if he did not govern well and fulfil his obligations as ruler. The Mandate of Heaven then transferred the right to rule to another person who satisfied the criteria i.e. his ability to govern well and fairly. 

Depletion of the soil, surplus food